I don't think ecash is trying to solve custody, it's trying to be the best form of custody.  Any custodial service is just a credit system.  I think it's well established that some form of credit system will have to exist in a bitcoin world since not enough utxo or block space for the whole world. 

Privacy aside, the credit system that is the most decentralized, or allows the most amount of people to manage small communities of credit on top of their bitcoin would be preferred over a centralized issuer of that credit imo. 

That's how I view cashu and fedimint, it allows many small Bitcoin community banks (custodians) to exist and issue credit that's interchangeable. (Not to diminish the privacy and cost savings benefits also)