But I also have ZX DOS+ (2Mb)
For myself, I make core under ZX DOS +
If you look at the SamCoupe sources from Sorgelig under both MiST and MiSTer, you can see that the joysticks are inserted into the ASIC module and their port is standard
wire kjoy_sel = (cpuaddr[7:0] == 'h1F);
The good thing about devboards is that if you want, you can use any joysticks, even from NES \ SNES ,
for example MD supports 12 bits as far as I remember.
The scheme does the rest