Oh, that's a terrible idea of a 'offline computer'. Whatever device you use for any such endeavor, should be either booted off a live drive (CD or USB thumb drive) or destroyed / at least fully erased after the process, and before any radios are turned back on.
How do you think criminals attack your device? Do you think a short downtime erases any viruses or spyware? Keyloggers and screenloggers continue logging - whether you're connected to the internet or not, and just do the telemetry when network connection comes back.

It's a little bit disturbing that 'turning on airplane mode' is what springs to mind when I talk about entering a seed phrase on an airgapped machine.

I'll be straight honest to you: Paper looks like the way better option here. Less fancy, but more secure.