The OS on the Steam Deck is Arch based, just like Manjaro, so I imagine it'll do games.
I'm a fullstack developer as well, and use Arch as my daily driver, and have for the past 9 years. While I can't speak for Manjaro directly, just the upstream, I have some coworkers that use it without issue. I think it'd be fine for your needs, at least worth trying out. I hear a lot of bleeding edge horror stories thrown around but in that 9 years 95% of problems were of my own doing, and the 5% were easily fixed with a rollback of a package. Out of that, my downtime isn't worth mentioning it's so negligible. I feel my coworkers on macos have more issues with major version upgrades by far.
On Arch-based distros, pkgbuild is a great way to handle custom packages when needed, and the AUR is gives me almost everything I need that isn't in the official repos. It's a great developer environment.
I'm very interested in OpenSUSE Tumbleweed as well, was thinking of trying it out as my next distro on a personal machine to try out something new since I've been on a single distro for so long, but not because I need anything new, just sounds like fun.
jq, or if I need to do something wacky a one-off python script.