I learned a lot during this project. Here's a list of my most important take-aways:
You can google anything, there is no secret knowledge only developers know. There is an extensive tutorial about everything somewhere online.
Color coding the folders you're using helps a lot in organizing, it is a smart idea to keep using the organization map I made.
ProjectCleaner really helps a lot with organizing, next project I will use this during the project as well.
I organize the world folder in categories (blueprints, meshes, VFX etc) and then subdivide those in areas if it's still a lot.
LODs help so much in making textures manageable.
Nanite helps so much in making meshes manageable.
Check every now and again how bad the lights are affeting the projects and if any of them can be turned off or baked.
Things that could still use some attention next time:
I don't fully understand how Nanite works yet. I want to do a bit more research on that.
I did not use culling for this project, because most of the time the whole level is rendered in. I'd like to learn how that works.
Overdraw wasn't too bad in this project, so I did not really look into that. I'd like to learn how that works as well.
In general I think I'll have plenty of opportunity to look into those points next project. And it already helps a lot that I know these are things that exist and I can check to help with performance.
But the most important thing that I learned during this project: I feel like I can finally talk about optimization with other people without pretending to know what someone is talking about.