Hi, this is quite a late response but maybe it still helps.
I'm not the dev but you can contact them at the options listed on their website: https://sophiehoulden.com/contact/
I don't know for sure about slimepires, but pico-8 games use a config file at C:/Users/Yourname/AppData/Roaming/pico-8/config.txt
In that file, there is a section called button_keys, it needs button scancodes, so it's not the most user friendly way, but if you can figure out what scancodes correspond to the buttons you want, it might work. I found out about this at https://pico-8.fandom.com/wiki/Keyconfig