I cannot safely upgrade to a higher version of Electrum without buying a new computer
Although I'm very curious why this is (even on an offline computer I'd expect Electrum's AppImage to work), this should also work for the older Electrum version:
In that case
the old method should still work. You can set
wallet.relayfee = (lambda: 0)
in the Qt console, and then the GUI lets you do whatever you want. Create even zero-fee txs.
Yeah, your method worked for the older Electrum version, I mentioned that specifically for the plugin.