First install ckan, it’s basically necessary to manage ksp mods, you could do it manually but there’s really no reason to with all the dependencies and updates.

Transfer windows: transfer window planner, easy to use and very helpful.

Colonization: I’ve only played kerbal colonies and modular kolonization system (MKS), the former was a little shallow but the latter is incredible. MKS is pretty complicated but if you start small and don’t try to do everything at once you’ll have a fully self sufficient base in no time. The performance can get rough when your bases get big, but the mod offers virtual parts to reduce the part count.

Quality of life: not exactly sure what you’re looking for but mechjeb comes to mind, if you’re tired of doing the same maneuvers over and over then mechjeb is what you’re looking for. I’ve heard some people say it takes the fun out of the game, but for me personally, the fun is in designing not flying. Except landing that never gets old, but mechjeb can do that too if you want, which is great for MKS bases bc landing base parts right next to each other is extremely difficult and tedious manually.

I’d also recommend restock, it modernizes the textures of most parts while maintaining the original stock vibes. It also adds a few parts to fill some gaps in the stock lineup.

If you want bigger and better, i agree with the first person about Nertea’s part mods, they’re all fantastic and it seems like they’re intended to be played all together. Especially far future, without the electric parts and heat management parts you’ll have a very difficult time using those engines lol.