Hi Gameblabla, thank you for working on these projects. I'm testing your SMS PLUS GX (jan 18 2020) build for colecovision at the moment. One bug, In the input remapping  wont map to buttons or delete your selection with L2 when actually choosing  to map a key to, but if you go to  and try to remap it maps your input button choice not to  but instead to  so there is an error there. When you remap  it maps your input button choice to  so it appears to be off by one. Perhaps the default should be X for  and Y for . It also should mention the L2 to remove.
2) When I exit out of the mapping, or enter the menu, it is showing whatever screen in the background behind the text. Maybe it should instead be black background.
Before I try, are the keys mappable to a usb controller or usb keyboard attached (using a usb c to a adaptor)? I just tested a wireless Logitech K360 keyboard works on the RG350 but it did not work unfortunately on the playgo (a black version of the pocketgo2). Another wired keyboard, Foldable Bluetooth Keyboard, Jelly Comb Dual Mode Bluetooth & USB Wired also worked. Good to know for other emulators such as computer based, apple, and others that need keyboard.
*EDIT1* It does work for the logitech k360 for key mapping to colecovision, but it shows ... for the key such as if there is no choice put it, but it does work fine in game despite that. Tested frogger using keyboard arrows to play and 1,2,3,4 for - to change the 1/2 player and slow/fast for example. Works good. How about adding save keyboard layout so there can be more than 1 and ability for mapping for the right analog stick.
*EDIT2* The L2 and R2 also map ok in input mapping (for example to  and ) but also show ... for the key choice such as there is no choice put in (It doesnt show L2 or R2) same as the keyboard issue. The mapping still works.