Since you're looking for the safest way to do it, I agree with @o_e_l_e_o's answer above. You only get closer to the highest safety/security possible when you eliminate all the potential risks and flaws out of your method. And so far, the only remaining potential security threats is someone physically tampering with your airgapped device which is imo a quite low probability unless you're being watched by intelligence agencies or have some bad actor around. That is unless you somehow mess up yourself and accidentally enter the airgapped device's seed into the online PC, which I believe is something that has a low chance of happening as well.

I would not use a phone because it has just too many backdoors and unknown/non-removable components. A PC is quite modular and you can easily remove or add anything you might or might not need. For me, privacy & security are not only interests but also a fascinating hobby (and now addiction, I guess). Even if you aren't followed by 3-letter agencies, I would still recommend to follow the best security practices as it's more than worth it over the long term. And since it's both fun and safer, why not?