Never tried Flatpak… I usually avoid Flatpak altogether. Multilib is usually just three commands from here http://www.slackware.com/~alien/multilib/ Then I just install my nvidia driver manually using the .run file after a kernel update before I startx (I’ve done it this way since 2002, never had issues).
Aside from that, I’ve never once had problems with games on Steam, or with Steam itself.
Poor dude has plantar fasciitis, he’s trying to stretch it.