mainnet wallet will refuse to dump privkeys

As the error message says, the type of wallet you are using does not support the dumpprivkey command. You are using a descriptor wallet, and individual private keys cannot be exported from the wallet.

You can get the master private keys by using listdescriptors true. This will output all of the descriptors stored in the wallet along with their corresponding private keys. Because Bitcoin Core uses BIP 32 derivation, instead of thousands of individual private keys, you will get descriptors which contain the master private key from which you can do derivation on to get the individual child keys.

Descriptor wallets specifically disabled export of child private keys because it is unsafe. Because unhardened derivation is used by default, a child private key and a parent extended public key is equivalent to having the parent extended private key. However it is not immediately obvious to the vast majority of users that this is unsafe, so child private key export is disabled.

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