Hello! Itch will come with Steam keys, usually devs cannot distribute on steam when their game has been released previously on another store, but when the release comes at the same time you are allowed to do it.
Anyway, the biggest form of support you can give a dev in general is still buying on steam, while it takes a minor cut, steam reviews left with a "normal" copy of the game count for Steam's algorhythm while copies got outside of the game don't (which is a big thing since Steam's market is hundreds of times bigger than Itch).
Anyway, thank your for your interest, hope I made the point clear for you!