mclinic

For those who care about the display firmware difference details, I've added to the first post of this topic:
Edit 16/Mar/2023: One concrete difference seems to be how the two chips send and receive I2C data. I2C seems to be used for the EEPROM chip and some other functions I have yet to identify. In the ST chips, there is hardware that implements I2C; in the GigaDevice chips, this seems to be missing. However, I believe that ARM input/output pins are very programmable, so it seems likely that they were able to use the same pins for I2C data, clock, and enable pins, and emulate the I2C hardware with "bit banging" and busy wait loops. Crude, but if you can't buy the ST chips, what else can you do? It's not like the display chips are chock full of time critical functions.
Edit 2 16/Mar/2023: When I went to look at firmware 12.21, which is supposedly able to run on both chipsets, I expected both hardware and bit banging code, selecting the appropriate one for the present hardware. But I just found the bit banging code. Then I realised: ST chips can bit bang just as well as GigaDevice chips, and the speed of both would be close enough for I2C, which from vague memory is more or less synchronous (each side waits for the  other to transition, except for the actual data bits). There may be more to the unification than that, i.e. it's possible that older 1xx.yy firmware won't automatically work in hardware that came with 1xx.zz firmware. But at least the I2C hardware difference is easy to address.