I've been considering joining a leftist organization outside of the DSA but have heard some pretty terrible things about both CPUSA and PSL. I've read some pretty glowing reviews that the APL is the only purely ML party that sticks to its revolutionary/anti-revisionist roots. However, I can't find any actual unions that support APL (besides a teacher's union in the Dominican Republic?) and their platform is very explicitly Hoxhaist, which I'm not very familiar with.
Has anyone worked with them before?
Out of those options I'd recommend PSL. It's larger and does things and its faults are plausibly reformable.
Also don't sleep on issue-focused left groups. They'll often be communists. The downside is that they usually have unique local names so you'll have to go find them.
What kind of work do you want to be doing? One way to find an org is to go to the events and actions you'd want to organize and finding out who organized them. For example, if you want to work on forming unions (presumably with a left angle), I'd recommend attending some pickets and start asking around to find some lefties.