Mobian has an amd64 image available. If you are looking for a “tablet” tablet experience rather than just desktop gnome with an on screen keyboard then that is going to be your best bet.
In terms of DE I would stay with GTK enviroments because GNOME circle has created a pretty extensive environment of apps that feel native there. Both PHOSH and GNOME mobile offer basically the sane experience so you should try them both and see, which you like more in the details
Edit: here is the install guide: https://wiki.debian.org/Mobian/x86
Looks to me like you are trying to update via fwupd. While this should work via GNOME Software, you can also use the the terminal to do this.
The command is
fwupdmgr update