I've suggested this before, but the problem is that I think get the vote among citizens to secede from a state requires a lot of work, even if Congress and the President are both willing to sign off on it. I don't think it's as simple as congress, the president, Gavin Newsome and the California legislature getting in a room and deciding how to slice California into multiple states that will each elect Democratic senators.
It's easy for us to say just fin 5 different heavily Democratic pockets in California and make each one a state, the actual people who live in those areas probably won't want to go along with seceding from California.