After 100ish hours played I finally decided to finish a game last night. It was 6 enemies on a medium single sector map, Genius difficulty. I got to the point in the game where dreadnoughts were taking 2 turns to produce, my early colonies had used all of their tiles. I'm glad you had an actual end game screen and charts. I play Stellaris as well and not having this is very disappointing.
The first 100ish turns are in a fantastic spot. So many decisions to make at every level of the game and each new tech/building/colonist seemed impactful. Also the decisions made on where to colonize, when to build colony ships, where to gather resources etc are all in a great place. I think you have really nailed early game.
Mid game gets bogged down more which isn't really unusual for these types of games. End game was a slog, I knew I was going to win at like 80% prestige and getting it up to 100% was just dull, I really feel this part of the game needs a way to accelerate the pace even if it means everyone attacking you. The rest of this post may sound negative but I'm just going to point out some parts of the game that I think might need work.
1. When planets run out of tiles for buildings the options for what they can do is extremely limited. I'd really like to see better ways to turn production into other useful things. I can only use so many modules.
2. There didn't seem to be enough attrition in combat. I took out 2 enemies and I lost less than an entire fleet in the process. Even fights that said I should expect light or moderate casualties tended to just cost me 1 ship.
3. The tech tree feels really bloated. Even though I was almost always taking techs that cost 1 or 2 I still had tons to research at the point that I felt like my colonies didn't have much room for improvement. Also the game seems to store unused research but doesn't tell you how much it stores, many times I'd research a 2 turn research and it would finish in 1 turn because of this.
4. Maybe give us a way to move structures at a cost rather than destroying them and rebuilding them. Trying to organize my colonies late game just felt bad. To make matters worse some buildings if you destroy them you can't rebuild them and the game didn't tell me which ones were that way.
5. I just used the auto generated ships which I know isn't optimal but at one point it only gave me ships that used special resources. I feel like you should always keep at least 1 auto generated ship that doesn't take any special resources for those that don't like to build their own.
6. Most of the things you buy with control didn't seem very useful, most games I just use the same 5 or 6 of these and the others don't seem to have a spot I'd ever use them. I feel like this system needs to be overhauled quite a bit.
7. It felt way too easy to keep everyone happy through diplomacy. The penalties for being different lifeforms etc seemed easy to offset. I didn't go for a diplomatic race in any way yet I was able to dictate when each war started.
I'll leave it at that so this doesn't get too long, I'm sure I'm forgetting a couple of things.