I had the exact same problem, also with SteamOS, and I, too, was trying to flash the SteamOS .bz2 directly instead of extracting it first and flashing the .img.
And I’m not alone! https://www.reddit.com/r/SteamDeck/comments/11uwwkd/balenaetcher_steamos_reimage_flash_stuck_at_0/
Lots of people are having this issue.
This seems like a bug in balenaEtcher. It shouldn’t just get stuck at 0% when I foolishly provide a zipped file; it should fail with an error message.