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.)