Is steam the only program you’ve noticed having issues? If not then here’s a scary possibility to consider: Hardware Failure
I recently had my power supply destroy my MB and SSD and there were signs that I ignored just before it all went down.
If you still have your arch install usb available then boot from that and pick the memtest86 option in the boot menu to see if your ram has any faults.

That trackball is one of my favorites, I really wish someone would make a real replacement for it. I’m currently pretty happy with the ProtoArc EM01 NL though.
I didn’t need any special drivers to change the forward/back buttons (on either of those mice), you just have to bind them in your system’s settings. If you’re talking about having program-specific bindings then xremap works well.