Is there any particularly reasonable explanation why you must use Bitcoin Core in your Air-Gap machine instead of another offline Electrum?
not answering for OP, but i would prefer to use only btc core for a cold wallet. is there a a straightforward and secure way to do this?
Certainly, but not as straightforward as Electrum Cold-storage but not too complex either.
The procedure are: create the offline wallet, export the descriptors, create the online watching-only wallet, import the descriptors.
You can refer this post for the steps:
http://bitcointalk.org/index.php?topic=5392824.0If you're using v26.0, the first step to select "
descriptor" isn't unavailable due to being selected by default; the rest of the steps are still applicable.
But instead of selecting only one script type, I'd suggest to import all 4 script types to the watching-only wallet, that's a total of 8 descriptors.