How to make a slash command only useable by a specific role?

Don’t think so, you’ll need to use an if statement whether you’re checking a database row for info you set for a user or if you’re using the get member block.

Add other slash command permissions has some info on why this isn’t a feature (yet).

You could also consider making a custom block that checks for roles if you find it annoying having to put the same blocks in lots of different flows.