Sorry for the newby misunderstandings, I finally figured out what you actually said (which is quite sensible but beyond my initial understanding).
Near as I can tell using diff tools this script is now what is being supplied with the downloadable tools for installing reTerminal drivers.
As near as I can tell, what would really be done in this situation (simplifying step-by-step):
Install the Bullseye image using the procedure here: [Getting Started with reTerminal - Seeed Wiki (seeedstudio.com)](Getting Started with reTerminal - Seeed Wiki)
Once this boots up the reTerminal has serious issues, like a black screen. But if the procedure was followed properly you can SSH into it from another computer and follow the procedure here: https://wiki.seeedstudio.com/reTerminal/#install-reterminal-drivers-after-flashing-new-raspberry-pi-os-ubuntu-os-or-other-os
This includes the new script as shown above. The screen now works as long as you remember to find out which port expander you have and add the line to the startup script if you have the old chip.
But the unit is still unusable. With an external mouse you can click on links etc. if you attack the left click button with extreme ferocity. Ordinary clicks are ignored. No clicks work on the touch screen. An external monitor is no longer useable. Orientation is wrong (portrait) until you physically rotate the unit up. Back to Buster for me using https://wiki.seeedstudio.com/reTerminal-FAQ/#q2-how-can-i-flash-raspberry-pi-os-which-is-originally-shipped-with-reterminal