Pretty much the same as any other incarnation of openwrt, just without a lot of the compatibility headaches and weird installation processes that you typically have with other architectures. It's just install and forget pretty much.
As for the link speed, you can just cat /sys/class/net/eth*/speed as with any other linux system. Not sure how your configurations stopped working or broke, maybe your storage got corrupted or something? Hard to tell, but I doubt openwrt caused it on its own, it sounds new to me.
Easiest way is to use heroic. You can easily install it from the discover store, log in with your gog account and click install. Then right click and add to steam directly from the heroic interface, this way it will show up in the deck game mode.
Ps: I know you wrote you don't need a fancy launcher, but this is legitimately the absolute easiest way to go about it.