CPUSA is kind of a special case in terms of "Real and once-productive org that has been 100% coopted". That said, individual chapters of any org are often way more radical than the head, so what you should do is research what is active in your area (PCUSA, PSL, DSA, FNB, etc.) and just pick whatever seems the best or most viable for you. Waiting for something perfect to emerge to hitch your wagon to is an exercise in futility. Very few of us like the central authority of any of these orgs, but that doesn't mean there aren't individuals doing good work in their community.