Thank you! Yes, the culprit were high-resolution textures and most of these were from Quixel. 8k textures for a single straw of grass is great, haha. I never shipped an Unreal game, so it took me quite a while to find the problem. I tried a lot of things shrinking the package. I deleted a lot of things from the levels and then manually from the project. I also tried to do some *.zip magic with high compression. I managed to bring it down to 3.88 GB, but that was not enough.
Fortunately a friendly dev at a discord channel gave me good tip regarding the textures, so I put some big compression on them all at once and was able to get the final build down to a comfortable size. I could even restore some of the stuff I deleted. :D