Could this work as an amnesic Qubes?

Would this work?

Install Qubes onto USB Drive then boot up setup all vms update everything and power off.

Then plug writeblocker between USB-Drive and USB-Port boot up and once done turn off and no writing changes to the USB-Drive have been done?

Only ever boot without the write-blocker to install updates preferably from a different network only ever used for updates.

Could this be a workaround to the last thing Tails is superior in (amnesia)?

That’s an interesting idea! Actually a Qubes, not Whonix specific question. Please ask on the Qubes mailing list:

Just wanted to note that USB drives with hardware write-protect switches are available. These really ought to be standard equipment for any techie…