@tamme I usually recommend either the Icebreaker or the Radiona ULX3S boards, which both have excellent open source support, and example FemtoRV "phantasy microcontroller" implementations in Mecrisp-Quintus:

https://1bitsquared.de/products/icebreaker
(Lattice UP5K)

https://radiona.org/ulx3s/
(Lattice ECP5)

Toolchain:
https://github.com/YosysHQ/oss-cad-suite-build/releases/