Do you have any recommendations for genuinely constructive communities focused on design?

I know what you're saying

But I don't think large communities will be helpful in that respect, imho they devolve into the design equivalent of locker room talk and most advice is design for designers 

Small groups are a different thing (like the cabal)

I don't find it very inspiring, to be honest. There are many challenges and feedback, but I want to connect with other designers in a genuine and helpful way. Many social platforms focus on flashy UIs instead of solid basics.

Then telegram / discord / google group with your clients :P point still stands