With Steam, it has really good controller support for most games and controller types. Even though most games only have native support for xbox controllers, Steam Input can provide support to translating most controllers to xbox commands the game can recognize. Even if the game doesn’t have controller support natively at all, you might be able to find a profile for the game that somebody has made (either by the community and even sometimes by the developer) that will map controls to output keyboard keys and mouse movements.
Also… sometimes you might have to go into options for some games to prioritize using a controller over a keyboard/mouse setup.
As far as RSI and related injuries go, finding what controller causes less pain is extremely important; even more so as you age. If I haven’t stuck with a controller for the past 35 or so years I’ve been gaming, my wrists would probably be much worse off.
If you do however want to explore the keyboard and mouse route, maybe look into a split keyboard or something like that. I’ve heard good things about them for the people that do use them for gaming.
edit:
I currently use this controller with mechanical buttons / dpad ( BIGBIG WON Wireless Switch Controller) :
https://www.amazon.com/gp/product/B0BYJVQHDM/ref=ppx_yo_dt_b_search_asin_title?th=1
while it only has Nintendo Switch (red LED) and Playstation (blue) modes, it works great for most games on Steam. Either modes are also picked up in Game Controller settings in KDE while using a USB C wire to connect. It also has a few other options to set its extra buttons to duplicates of others (great for things like R3 and the like when you have to press the stick in; as with using the analog stick and trying to do that doesn’t feel great to say the least 
)
If you are looking for a wireless gamepad for steam, the GuliKit KK3 MAX Nintendo Switch controller works great for Steam. The wireless dongle it uses seems to work fine on most PCs too (far better than trying to rely on bluetooth for connectivity like most controllers).
Edit 2:
Found a useful guide with using Steam and Controllers:
https://partner.steamgames.com/doc/features/steam_controller/getting_started_for_players
The guide is a little dated as it doesn’t mention Nintendo Switch controller support (which Steam has had for a while now), but most of the information should still be useful.