G-code Reference for Snapmaker - Google Sheets is published by a Snapmaker Engineer. Knowing how much attention they put into v1, I’m sure they didn’t add anything to it after publishing that list. If you really want to know, the v1’s firmware source is posted on the forums.
I can’t find the post, but I recall reading that the v1 does not support any sort of interactive pause. It supports the G4 command, but I haven’t verified that. I haven’t seen anything that would allow you to put text on the controller screen.
I only read GCode, I haven’t attempted to write it yet. But I think a combo of G90 (absolute positioning) and G92 (set position) should get you able to drive all axis against the end switches. brent113’s G28 suggestion is also a good place to start. Give them a try in the serial console.
From what I’ve read of the v2, the v1 is much more solid and repeatable. No tramming issues and no random bed warping. I re-level mine about once a year, usually because the thumb screws holding the bed on needed to be tightened. The only time I’d call my machine “shaky” is when it’s doing the layer fill on the first and last couple of layers.