In IanColeman's BIP39 Tool; your binary seed is equivalent to that tool's "BIP39 Seed" that is derived from the BIP39 mnemonic.
Your Master Private key is equivalent to that tool's "BIP32 Root Key" that's derived from the above.
Additionally, your watching-only descriptor's xpub (extended public key) are equivalent to the "Account Extended Public Key" of each address type's specific derivation path.
Thank you again. It's really a great help, even if I've run out of Merit.
Would the Core Client fail to create the wallet or issue a warning or block if it does not have enough entropy available when generating the binary seed? Does it read the entropy from /dev/random or also from /dev/urandom?
I'm on some kind of embedded system, without a fine-grained clock, with no network, with little inputs, reading from flash which much more deterministic timing for operations. I therefore depend on the hwrng of the Raspi. Hence my concern. Unfortunately dieharder is currently broken on the 32-bit Raspis under Bookworm. I can probably fix this and have other testing options.
In case there is a developer reading here: For the future it would be nice to have the option to pass a binary seed to the Core Client when creating a new wallet. Or to have something analogous to sethdseed for the new wallets. Creating a Master Private key and descriptor yourself is not necessarily what a beginner wants to do. But rolling the dice and handing over a binary seed (in a denfied format) is a minor intervention which requires only general computer and mathematical skills, no specific knowledge about Biicoin.