Thank you for your reply! I did a bit of research and, since I do want to use quite some peripherals, I have gone for the ULX3S. A big factor here was documentation and availability; If I would have been able to find a MiSTer Pi I might have gone for that instead.
I now bought a ULX3S on a whim, and will at least evaluate how usable it is for my purposes. It will take quite some time to familiarize myself with a new toolchain, which kinda sucks. One advantage of these big proprietary IDEs is that they integrate a lot of functionality into one "unit" (as far as the user/programmer is concerned), instead of having to install a lot of separate tools.
For the course, I am now considering to "support" an AMD board, an Intel one, and a Lattice one.
Good choice!
oss-cad-suite will give you the open source toolchain for ULX3S in one convenient package. There are plenty of example projects and other resources, plus a discord server. https://ulx3s.github.io/
(Also, to download Lattice Diamond you'll need to make an account on the Lattice website which then needs to be activated. I tried that using a gmail account, and it was never activated - I had to use an email address related to one of my own domains.)