Oh, that's a terrible idea of an '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 your mind when I talk about entering a seed phrase only on airgapped machines.

In my opinion, the app should not even exist and the NFC transceiver should come by default. You could provide a modified version of Tails with NFC drivers and your own software preinstalled; though do make sure that it's all open-source, with reproducible builds.

I'm slightly worried about the NFC device though: where you source it, whether it's trustable. Though I suppose if it's not plugged into anything other than the live-USB-booted computer, there's no way for it to do telemetry.

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