Thank you all for putting me on the right track once again. And also for the references to the specific algorithms.
If this is about finding an easy way to backup your Bitcoin Core wallet to a paper or metal plate,
it should be done the other way around (I've seen your thread about creating cold-storage Bitcoin Core):
Create a BIP39 mnemonic->Derive the Master Private Key->Manually create descriptors with it->Import to a blank wallet
Yes, it was a bit of a XY problem. I actually thought about creating a mnemonic from the master key, which I have now learned is impossible.
But the obvious downside is you'll have to use a third-party to generate the entropy which could be using insecure RNG.
So try to find a safe open-source BIP39 tool/wallet that you can audit if you really have to use a mnemonic seed.
Exactly, that's why I don't want to do that. With what little knowledge I have, I prefer to handle things canonically (although I am only just learning what that actually means),
You can also just backup the master private key and your descriptors' various derivation paths if the whole descriptors are too long.
I will back up the wallet offline but digitally. Nevertheless, I would like a paper back-up in case of disaster. It should be sufficient to note the master key and the root of the path of the descriptor used (for example /84'/0'/0'/0/*), right?
The public part is stored in my watch only wallet (full node), which I use to create the unsigned transactions, which I then transfer to the cold wallet and sign there. That's how I've understood it so far.