I've almost ported all the mods I made to KLIC to FLENG and upstreamed them. It was quite painless, by which I mean done in library code without needing to touch the compiler (unlike KLIC). The FFI supports stub generation which is handy, and there's a separate repo to more easily contribute things like bindings. This aligns with my view that before writing a new Blub library, check that there aren't already standard UNIX functions to do the same thing.
Flenglibs, libraries for FLENG
Also, any ARM64 bugs I've found in the main compiler have been fixed promptly.
I may not be the most social, but in terms of "lessons learned" I think the big one is that some minimal level of running a project/community is basically required now. There's no need to print T-shirts, run conferences or look for donations. But at least offer ways to contribute like a git repo, defect tracker, and accept pull/merge requests (some people dislike github for ethical reasons, which is fine, there are alternatives). I also note that lots of FSF projects get good results with mailing lists & IRC (although some other projects get worse results in my opinion by using non-standard tools&services). However, just publishing tarballs is probably not sufficient any more even if it once was.