First of all thanks for a great game, having lots of fun!
After playing 130 turns in the Imperial campaign I noticed that at some point the defensive structures become obsolete/near useless.
Especially the turrets who already in early midgame become near useless. I only use them for extra scans (and as bait for the stupid ork/tyranid AI that rushes into them so you can easily spam AoE-stuff ontop of their tightly packed fleet, but this is more an issue with the AI).
Turrets don't even seem able to beat frigates! Starbases are still quite alright, but they are also quickly falling behind as the fleet sizes continue to grow more and more later on in the campaign.
My suggestion is to allow us the option to be able to upgrade both turrets and starbases (perhaps unlocked with the tech-tree, so that you cannot do it until later) to improve their efficiency with upgrades or multiply their numbers.
One other important suggestion would be to allow turrets/starbases to count as a force when invasions arrive. I had one occasion where an 800 Dark Eldar invasion-fleet captured Medusa from me, even though I had a starbasea and turrets there because I forgot to move the fleet above that was guarding the Ork-system. It's quite possible that if we had the option to fight with our defenses (at least in auto resolve) then the Dark Eldar would have lost a modest amount of ships, and if you added the option to upgrade and/or multiply defenses then they could even hold systems on their own. But as it works right now invasions seem to take zero damage even if you have a starbase present.
In regards of mines it'd be interesting to encounter them in actual battles, not just as a passive system buff/debuff on the campaign-map.
Perhaps also attainable through an upgrade, like with the turrets/starbases in my suggestion above. It would be interesting to encounter stealthed proximity mines that would be pulled towards your vessels when you get close enough, and your defense turrets would have to deal with them.
Or give interceptors a second role in being able to clear minefields if you for example send an augur probe into a field. This would open up new strategies in battle!
Thank you for reading! Looking forward to more future content from your team!