In order to use nested tmux sessions, I change my prefix key to Ctrl + a for my top-level terminals such as Termux.
set-option -g prefix C-a
Then for servers that I ssh into, I leave its prefix key as the default Ctrl + b.
I find this makes intuitive sense, as C-a controls the bottom-most session, then C-b controls the one above it. If there were a server that I would commonly ssh into from my C-b session, I would use C-c.