If you are going to use something like Electrum or iancoleman on your airgapped device to import your hand-generated seed phrase to give you an address to send to, then you could skip calculating the hash for the checksum altogether and just brute force it, as both of these services will tell you when you are using an invalid checksum. With the first 3 bits of entropy already known, there will only be 256 possible words.
everything else can still be done with a computer after the number was physically generated using a coin or something like that.
You still need to be sure that the software you are using isn't just spitting out pre-generated addresses regardless of what seed you enter. You could go through the process of performing each operation from seed to address manually, or more simply (as Loyce has said above) is you could import your seed in to multiple different wallets (all airgapped of course) and ensure the generated addresses match up.
It'll complain that it's not a valid BIP39 mnemonic. I know Electrum will let you bypass that and go ahead and use it anyway... but surely for max compatibility you'd want a "valid" mnemonic!
Yeah, there's no good reason to settle for an invalid checksum. If you input a 24 word phrase in to iancoleman which has an invalid checksum and then click "Show entropy details", it will automatically change the final word to the correct checksum, maintaining the same 3 bits of initial entropy. Doing so will obviously then lead to a different wallet with different addresses, so can only lead to more confusion down the line.