Can't comment on Mercurial, but "for all my personal project where I don't need to care what anyone else thinks" I am using Fossil. Ever since that decision, I've felt a bit, well, held back, or rather, I don't feel quite as comfortable as I do at home when I have to use Git.
I've always been interested in Fossil, especially how they handle all the things in a project that aren't strictly code but still need to be tracked.
I'm like the person you responded to, I've just used fossil personally for years after working at a place using it a while back and always liked it.
This is its moment though. It is so well suited for LLM coding tools. You can jam all the markdown context, skills etc into the wiki. The CLI has wiki and ticket tools so those are just available for it to use. Fossil does not mind if you use the repo DB for your own stuff, so you can log all your sessions in there, fts5 is plenty for as needed on demand retrieval.
Big changes to professional development over the last year and hard to predict how it will all shake out, but I think the tooling will converge on something that fossil already has all the structure for. I was a late adopter on LLM-assisted coding but already feel ahead of a lot of my peers because of how easy and effective this approach is.