I think you're on the right track. You recognise that your mother is out of line, which I agree she is. Limiting what you share reduces giving her ammunition to use against you. Limiting time you spend with her is a strong measure, but may be necessary to enforce your boundaries.

I would be careful about issuing ultimatums. If you value your relationship with your mom, you have to understand that change is hard for most people. Still, you don't have to stay in the line of fire, when your mom is taking pot shots at your choice of a partner. Definitely, stop engaging with her, when she says things like calling your boyfriend "useless." You don't even have to spell that out to her. Just do it consistently, and she'll get the message.