How about something REALLY crazy.
We already encountered issues with "mega-chokepoints" hampering the depth of the game, with AI turrets. AI's used to pile on turrets leading to similar sorts of degnerate situations without any sort of way to turn defense to offense when needed. For this reasons (and several others), turrets for AIs were eliminated and guardians were introduced.
Now we are facing a similar situation with humans. Why not a similar solution?
Just completely get rid of immobile military turrets (except maybe snipers, though maybe also get rid of some non-military ones, like tractors). Replace them with human guardians.
To prevent them from becoming bascially like starships, maybe:
They can be built directly from command stations/mobile builders, or at least there is a high ship cap, cheap and easy to defend structure that can build them.
They would be able to use wormholes.
They would inflict half DPS when outside of supply, to encourage that they are meant primarily for defense.
They would take 1.5x to 2x damage when on an enemy planet, and another 1.5x when not in supply, also to emphasize that they are primarily defense units.
There would be a new option that would basically be "Cowardly guardians", where it does the opposite of "Brave starships" for gaurdians, even though they are mobile military, when this is checked, they would act as if they were turrets in terms of selection behavior.
Some sort of auto-rebuild mechanism, such that if a guardian was lost on a planet, one dies, and there is something that can build human guardians on that planet, it would automatically be queued on that planet. There would a control option to turn that off so you could manage it through "standard" means like loop build + garrison control option.
Again, crazy idea, so crazy I would not support it, but it would solve many of the concerns here.
How chaotic would it be to make MK III turrets take 10x as long to build, but get 5 speed and the ability to jump wormholes?
Is it possible to have any possible mechanic where a shot from a unit on one end of a wormhole in some way effects something on the other end of wormhole (through shooting into wormhole?)
This is another idea, instead of gutting turrets, make the highest mark act sort of like a human guardian as proposed above.