OK, I'm not a C coder, I used java for a maths PhD. I've built LFS and I've managed to put Linux on this phone (Motorola G13) without rooting

Maybe I'd be up to porting ?

I've also got an Honor pad x7, I'd like to lose android (and Google) on both

I suspect the answer to this will be "no chance because neither device has a USB that will flash the ROM"

Can I use Termux as the host and then one device to flash the other?

(I guess that's also presupposing that I've got something to flash, to which the other answer will be:- you wouldn't have a hope of doing that without a Linux PC)

?