I just have in my personal systems ls aliased to exa (with some of my preferred options like group-directories-first) and on other systems it's still ls. So my usage is the same regardless of whether it's exa or ls. This isn't a programming language or a complicated tool like jq vs competitors where what I need to do changes. I just get the nicer presentation on my local machine and "graceful degradation" back to the defaults of GNU ls on remote machines which don't have my preferred setup installed on them.

If you do that consider if command -v exa > /dev/null instead, so you don't spawn subprocesses unnecessarily.

Exa is an interactive utility. It doesn't make much sense to put it in a script like that. If you were to use rg or ug instead of grep, branching like that would probably be worthwhile, though.