thanks for your continued work on this game. I'm new to the series but it's nice to see devs responding to the community, and you seem really passionate about improving the AI and gameplay.
Though I've had only a little experience so far, I think there some changes required from a balance standpoint, and that many of these would help streamline the focus for the AI.
The most abusable power differential right now, as I noted above, is move speed. This I have found gets even more egregious later into the game, due to the ability to stack support modules (another questionable decision.)
In my current game, I am only in the midgame, but my planet cracker fleet has 50 move (as well as outgunning all opponents by a factor of 10) due to stacking move speed enhancing fleet support modules. This should not be possible, except maybe on Ludicrous sized maps. 50+ move in a turn based game like this completely destroys strategic balance. The enemy cannot plan and respond to threats, because I can literally hit all his unguarded starbases and planets in a single turn. Positioning and scouting are rendered irrelevant. Even if both sides were taking advantage of this exploit, the game would not be balanced, as tactical decision making in a tile based game relies on the ability to predict your opponent's options and respond. You cannot do this when your opponent can cross the whole map and hit any target in a single turn.
So, step one is to remove fleet wide support module stacking, and tune move speed bonuses based on map-size. for most map sizes this should mean a severe nerf to available move speed bonuses.
This will help the AI out in several ways, as I'm sure you know. First, it will penalize them less for not having the correct ship designs that abuse move speed. Secondly, it should improve tactical movement decision making, as fewer options=easier AI optimization. Finally it should free up system resources from being wasted on long pathfinding algorithms (problematic on larger maps.)
The second major change I would suggest is around invasions.
The fact that you can capture planets costlessly if they have no legions stationed there is not good for game flow. It is much too easy to get an early lead with a blitz attack and a single transport, especially given how expensive it is to station a legion on a planet in the first place -- you have to create a special structure and build a legion or spend a citizen on a general, then spend more production to garrison them. Defense is more costly than offense, AND less flexible. This shouldn't be the case.
My suggestion: give population some resistance value equal to some fraction of a legion, say .25. However, they don't gain benefits from Soldiering technologies (meaning the ratio of population to legion favors the legion more the later the game goes, this prevents large populations from being too hard to capture late game).
In addition, make legions cheaper to build. 750 is too high, given they are necessary for early game defense. Instead, increase the cost of the transport module.
Overall, the impact of this is to make planets harder to crack, as they will have higher defensive value from population, and more legions available for defense. Additionally, casualties on the attacker's side will slow down momentum rather than allowing rapid fire jumps from system to system.
Combined with the lower move speed of ships, blitz rushes of enemy civs will be severely slowed down. In all my games so far, I've been able to wipe out an enemy civilization in just a few turns with 1-2 transports and one planet cracker fleet. this should not be possible. It should take a couple turns per planet, per transport.
This will make combat more of a tactical slog planet to planet, rather than a smash and run blitz.
These changes are significant, but necessary in my opinion. The game as is has so much promise, but is utterly broken to the point of being trivial for anyone with 4x tactical expertise.
I hope to see your continued support of this community and game pay off!