1. There is no interaction between citadel's owner and enemy fleet, if citadel vulnerable only 3 hours in week
2. How huge fleet needed to destroy citadel with more than 1 billion effective HP (if will be 99.9% omniresist instead full invulnerability)? You need a lot of interactions to organize such a large fleet

To be fair, Citadels are planned replacements for outposts. WH space currently has no equivalent and the argument for WH dwellers is why would you bother with a structure that allows you to accumulate more things than you currently do? Is J space the same as K Space? K space has static gates and cynos and other force projection mechanics in place. J Space has dynamic entrances that are a natural protection during the vulnerability window ( 3 hours a week, you do nothing but roll holes, don't bother with guns ).

These structures are not intended as a POS replacement, they are an outpost replacement ( which you currently can't deploy in J Space ), so answer a simple question, K Space has stations/outposts which have asset protection by nature, what equivalent structure exists for J Space?

I honestly don't feel it's fair to WH Dwellers to allow these to be deployed in J Space at all. It promotes accumulation of junk, gives a place to death clone to and also breaks the new(er) Shattered wormholes as they become available to be occupied.

If I kill a WH dweller in null, and get the pod, he can now reship and be back on my doorstep without having to find a way back to his home WH?

I really like the 'built-in Target Spectrum Breaker' idea of Skia Aumer, soft cap as opposed to hard cap, and can be implemented using something already in game (though would need some tweaking I think).