Ah, but, such a huge investment of resources should automatically get special treatment, so just having a siege window when the safezone can be dropped is a "massive vulnerability" that needs a safeguard. I actually completely forgot that the safezone itself is a huge protective gimme and took it on its face. Considering that, the idea of adding even more mechanics to protect the protection does seem like it needs to be called to attention.
EDIT: New Mechanic for Inhibitors - Sanction Vouchers
Concept: besieging a station gives the offending company or faction a debuff making it easier to besiege stations they own and/or debuffing their ability to make several sieges in rapid succession
Effect 1-Voucher Application: anchoring an Inhibitor to open a station's Siege Window applies a debuff, called a Sanction Voucher, to the company or faction that owns the inhibitor. This Voucher lasts for an amount of time between one day up to a week and if a stack of Vouchers accumulates only one decays at a time.
Effect 2-Tyrant Debuff: companies/factions with at least one Sanction Voucher take longer to anchor their Inhibitors, during which time the device remains vulnerable to attack or capture- once anchored the device becomes safezoned until the Siege Window begins, as normal.
Effect 3-Reprisal Buff: stations owned by a company/faction with at least one Sanction Voucher are more vulnerable to being sieged, Inhibitors anchor on their stations faster and the Siege Windows last longer.
Special Effect-Mercenary Contracts: if a mercenary group is paid to capture/destroy a station they can take a contract with the buyer, the contract determines up-front costs, time limits, and final payout and insurance coverage for the mercenaries; however, when the Inhibitor is anchored, the buying company/faction that hired the mercenaries gains a Sanction Voucher instead of the mercenary group. While a mercenary group can take the station and then turn it over to the buyer, this contract rule allows them to secure payment and ensure terms are honored by the buyer and at least discourages the mercenaries from being the target of reprisal attacks for simply taking a job.
Special Effect-Brinkmanship Cancellation: Sanction Vouchers are tagged with the company/faction offended by the siege, if two companies/factions have Sanction Vouchers belonging to each other they cancel out their effects on a voucher-per-voucher basis. This special effect ensures that two powers can be mutually belligerent with little disruption of back-and-forth sieges, slows down a very aggressive player that deploys large amounts of inhibitors in rapid succession, and also dissuades a company/faction from aggressively besieging multiple companies that are not part of the same faction at the same time. Additionally, it incentivizes alliances between factions/companies as they can benefit from a greater buff/debuff ratio than the defending faction in tit-for-tat warfare and allows even a small company to become a large asset in a long-standing war where vouchers have built up on both sides.
EDIT 2: thinking about the anchoring process, instead of a corrosion aura or whatever, what if the inhibitor created an unsafezone aura of several hundred meters around itself until fully anchored, meaning the whole civilian shell exploit wouldn't exist (at least until it became possible to make ships bigger than the unsafezone...) and something something minimum anchor range is at least 1km to prevent abuse against stations and something something inhibitors self-destruct if they enter a safezone not owned by their company/faction which would prevent further station abuse or company-hopping an inhibitor to avoid Sanction Vouchers, etc, it'd also encourage companies to keep their own inhibitors away from their own stations once built to clear its interference with their own safezone, making them vulnerable assets to keep around and stockpile (what if an inhibitor creates an unsafezone while being constructed at a station allowing the facilities nearby to be sabotaged by infiltrators, oooh, that sounds kinda cool- it could even grow the zone in radius as the component modules are assembled around it to create a ramping vulnerability)
EDIT 3: TL;DR- Lots of Mechanics Ideas, Very Dense Writing
Core Concept
I'm gonna elaborate on the unsafezone a bit more because it also seems kinda interesting. So, an inhibitor needs to be constructed, probably out of several modules that generate each effect, from anchoring, having ownership/being capturable, opening the siege window, and being invulnerable while anchored, and then have additional structure and plating installed around it for protection, power, and movement. Let's say each module combines to create the total unsafezone, and as each module is built from the anchoring core up to the invulnerability field, the size of this unsafezone grows in 50m increments. So as each module is completed more of the host station around itself becomes vulnerable until the inhibitor is finally completed and moved away.
Construction
Now nothing tells anybody that an inhibitor is being constructed outside of this growing bubble of cancellation within the station's safezone, so a well-built station might prevent anyone from getting near the spot where the inhibitor is being built- but regardless, an espionage payer or inforbroker could wind up finding out, etc, etc interesting gameplay options here, and a lightning raid or sabotage mission could be launched against the facility to destroy the inhibitor and/or damage station sections within the unsafezone. It's worth noting that I envision the inhibitor being constructed in some kind of station lot where you begin with the anchor module and the lot automatically adds the blueprint hologram for the next module. I'm assuming these four modules (anchor, ownership, siege, invulnerability) are fairly large and will take a few minutes each, minimum, and a large amount of materials to fill in with build tools, so the exceptional durability of the inhibitor iirc would be based on the sheer amount of materials it takes to build and the multi-part nature of its construction.
Corrosion Aura Redux
If voxels limit how much material can be in a single unit of space, I'd add optional "shielding" modules around the anchor, siege device ("siege beam"?), and invulnerability field and have each emanate corrosion while each part is active- first this would necessitate adding the shielding to block the corrosion coming from the modules; two, make the build time a variable choice depending on how much durability you wanted; and third, make the inhibitor more vulnerable to damage the longer it operated, so anchoring to siege a station and then having to wait longer than anticipated for the window to open would add a nice little set of variability to the attackers planning, further, taking longer to end the siege means the inhibitor becomes increasingly vulnerable to damage, with or without the shielding. This feels like a way more interesting take on the corrosion aura in the original idea and less like some gamey exploit-prevention- it also adds important long-term choices to the attacker instead of just pumping out inhibitors that always function in an identical manner there's a range that can be selected from and sectionalised durability can be mixed and matched to meet a conqueror's needs- I find this element really, really, interesting as a player and designer and hope you give it due consideration.
Espionage Gameplay Opportunities
Building inhibitors in a lot feels intuitive and puts a range limitation on attackers, since they will have to move an inhibitor from its build site to the target station, which means infrastructure and locality are important factors on the strategic layer of planning sieges. Further, because the "unsafezone mechanic" alleviates cheesing concerns, the inhibitor itself can be freely built upon in any way shape or form and it can be a towed object or have its own engines, weapons, and maze-like armored corridors built over it with little worry about exploits- it's a big, awkward, and heavy thing that will slow down and eventually immobilize any ship it's made a part of, we already know that makes it vulnerable enough during travel and while being anchored. Having the unsafezone while being built leads to potential espionage/infiltration gameplay potential which is actually really hard to create in most games.
The way this idea is shaping up it'll be emergent espionage dependent on the skill of sleuths, station designers, and company security and not some instanced and predictable simulacrum. It'll be difficult for spies to find inhibitor build sites, which means the station owner only needs to worry about making sure the build site is hard to access, whether that means it being a remote or private station, or smart station design and walled-off areas on certain company members can access, or an active guard rotation and dedicated counter-intrusion operatives looking for signs of infiltrators- passive defenses and the limited telltale of an inhibitor under construction mean this whole section of gameplay won't become a second life for players involved in it; lastly the exterior structure around the inhibitor can be customised in light of any worries about spies. etc, to make the build process faster/less intensive/more dependent of fleet support to mitigate how long an inhibitor remains vulnerable in its construction lot.