I've had some thoughts about it too. In addition, I think, entities which want a piece of your data, should be able to somehow define "queries" and the "custodian" (in your terms) should be able either to tell yes or no (if you accept to answer to the query).

I imagine it this way, you have some kind of device, wanna drink in a bar? The bartender can "query" whether you are over 21 or not, you see the query, authenticate (with bio and pin/password) and your device confirms that you are over 21.

Wanna take a credit, the entity queries to check if your credit score is over 9000, and if you agree to let them know then your device just answers yes/no.

But there must be ways to protect the data, ideally the data should be encrypted at rest (wherever) and decrypted only in your device in some kind of secure enclave, so that your custodian can sign the result of the query being sure that it is not altered.