Hi all!
I have an update regarding the usb issue.
I've contacted the vendor of the RCM x86 from tindie and he told me that the usb issue is related to the switch FW version (7.0.1 in my case)
He told me to try with these files on the modchip: github /euclala/RCM-X86/tree/master/ex_SD_reload (cannot post links, sorry)
flash the UF2
x86.load on the sd root
and a payload.bin on sd root
With these files, usb runs without problems: I've installed a 8gb nsp without issues and I'm able to inject payloads with tegrarcmgui.
@mattytrog from the comments on github, seems like you helped the dev with this code. At least in my case, this code seems to do something that correctly sets the USB interface.
Hope that helped. Thanks!