Hello!
Did you went for Focal 20.04 or for Noble 24.04? The second one is a little more buggy because still in development, tough almost ready for a stable release from what I read recently 
I tried once and didn’t succeed in booting on the SD card. If you manage to do that I’ll be happy to follow your instructions 
Hmm, never encountered that… Did you try a different cable? Did fastboot devicesalways returned the device when the other fastboot commands didn’t react?
I have the same (and consistant) behaviour with TWRP image, as I reported here. I’m not able to fastboot boot twrp.img (it just reboot and nothing) while flashing fastboot flash recovery twrp.img works flawlessly…