Happy to jump on a call to share my thoughts. I built misthos.io in 2018 to solve shared custody. But didn’t gain traction and ran out of money. Have thought about it a lot.
The trickiest thing to solve for is when members of a group change (think an wallet for a company controlled by CxOs but one of them leaves/is replaced).
If you solve the UX for that scenario (ie seamless key rotation) you’ve got a killer!