Very good summary of alternatives @dgalland.
Additional information for consideration.
UVC
Another option some are exploring with… using UVC. Here is a facebook post on a project.
I think UVC has bit-depth limitations… so not my prefered path. But for many it may be sufficient.
Stepper alternatives to Raspberry Pi
My first scanning round was with digicamcontrol and arduino (coded in C). The gate that I used (Canon S400 projector) had a microswitch to detect the end-of-film, so it would stop by itself.
This post also provides an alternative to stepper control with the PICO. Simple USB-serial control of the stepper. @Manuel_Angel developed his own as a workaround to the RPi5 lack of pigpio.
The above adds alternatives to the Canon SDK or Digicamcontrol without RPi pigio. In both cases, the movement/light control is down to a USB wire (Arduino or PICO doing the work).
No QT for me
In the latest project I also made different choices from your analysis, and would like to share why, in case is helpful.
For those who know QT, it is a very powerful option with many advantages… but it is proprietary and the commercial license limits what one can do. It is also heavy for a RPi4.
I instead chose GTK4. The main reason is that it provides memory-to-display with zero copies. Using shared memory allows to present it with less CPU overhead. That is how I can show all the gui components -with the remote VNC overhead- running everything in a RPi 4.
And… it is not proprietary.
Understand in your breakdown the RPi + HQ is a special case, which this last comment falls under.