I mentioned Loro in another comment, but they actually do conflict resolution on top of Git trees [0]. Jujutsu is also interesting but I'm not sure if they do any conflict resolution [1].

[0] https://loro.dev/blog/v1.0#loro-version-controller

[1] https://github.com/martinvonz/jj

More like Pijul or Darcs?

Git is popular, but it's not particularly great at conflict resolution.

For all things Git is good at, conflict resolution is most definitely not one of them. There are many ways to slice a conflicting diff into plus and minus parts, and somehow Git usually manages to create conflicts that are the least human readable. We live in an era of widely adopted and universal language servers, yet Git pretends that every change is just a char added or removed. There are many tools out there which do a considerably better job at this, for example by diffing the AST, not the text representation.

Hi, I'm a hobbyist coder with lots of personal forks of open source projects. I frequently spend a lot of time merging in changes from upstream. Can you suggest tools that would help with this?

The advantage that CRDTs have is they have a complete replay. In theory you could do some clever stuff with that.