I’m biased as one of the authors of Y-Sweet, but I think Jake’s choice of Yjs + Y-Sweet is as good as any to start with. We’ve worked a lot at making the localhost environment easy to get started with, and Yjs is the de facto CRDT with the most written about it.
I'd like to start with something that's more DB oriented like SQLite both locally and in the browser for example but I understand a documented-oriented approach might make more sense.
RealmDB is sort of the goto. They were acquired by mongo which could be a negative signal but the team is solid and I haven't seen it result it much degredation.
EDIT: actually... it looks like mongo may have just announced the EOL for server-side component a couple weeks ago... bad timing!
https://github.com/jamsocket/y-sweet