For me, I use Spacebar as a leader key in Neovim, so I think they would conflict.

Does it ever create issues for you if you need to type the backtick in the terminal?

I found that Ctrl-A does a great job: it is conveniently located in relation to other keys that I need to interact with after I activate the prefix, and is in general easy to use.

Ctrl+A conflicts with readline's "go to the start of line" (unless you're using vim key bindings in your shell, which I find uncomfortable).

I've been using Ctrl+Q, it replaces an almost completely useless key combo (flow control if I'm not mistaken), and is easy to press.

I also use backtick for my tmux leader key. It doesn't create issues for me in the terminal (but you need to tap it twice when you want a literal backtick). This became natural for me pretty quickly.