It would be much more practical to support mnemonics in bitcoin core, at least for import, instead of using xprivs in the descriptor, we would have an extra function to import a descriptor containing a mnemonic, e.g.:
importdescriptors '[{"desc":"combo(bip39([phrase1,phrase2,phrase3...],passphrase/0h/*))"#fingerprint","timestamp":"now"}]'

By the way, is there a safer way to get the xpriv root bip32 to import into bitcoin core as a descriptor without using iancolam.io, since it is an html file that opens in the browser?