You and nc50lc helped me again a lot. Nevertheless, I need to look into it more myself in order to fully understand what you have written.

What I generally understood:

The process of signing may require significantly more data than appears later in the transaction, as previous transactions may also be included here. The PSBT may then have to contain this additional transaction data. The size of the PSBT depends on how my transaction was added to the blockchain. I could use Taproot (Bech32m) to avoid the problem.

An average transaction is only 300 bytes in size. I could therefore mitigate the problem by converting the signed PSBT into a raw transaction on the offline device. Then at least the return path via a QR code would work. Do I see that correctly?

I don't know what you think, but I'm a little reluctant to use Taproot because it's still so new and I know so little. I didn't even understand the normal Segwit. I think I'll stick with the Bitcoin Core standard addresses (native Segwit) and figure out how to deal with the big PSBT problem and my air gap later.