Russell shouldn't sign for a lower team than the ones in top 4 right now. As there is no room at Ferrari and McLaren he should just continue his career at Red Bull.

Cadillac... I don't know about this. We literally have no idea about how fast their car will be right? For now at least. I don't think Russell would like to take that risk. He is one of the best drivers on the grid.
There hasn't been any "new" teams added to the squad in a long time, so adding Russel for that team would be a great addition. As far as I know, Sainz was in talks with them, no idea what happens now that he is in Williams, but if he can keep the deal then he may go there, so a George plus Sainz duo could be all a new team could hope for, and I bet that they won't be that great on their first year, who could be, it's a tough thing to build a great car right away.
However, if they have George and Sainz, those are racers that can tell them what is wrong and because of that I am sure that we are going to end up seeing them get their car better with time, it shouldn't really be a problem at all, they should be getting better results and get the car better with ease, thanks to that experience.