If have a reliable custodial service you can create an OAuth login API and the rest of the features you want and we can integrate your wallet into Cypher Box. It supports self-custodial withdrawals and top-ups to built-in hot and cold wallets.
Discussion
i'm at my limit and relying on third parties
This is the problem in bitcoin. We need more reliable custodians, not less. This whole space is fighting over stupid shit that nobody gives a f about while discouraging individuals and businesses to run custodial services with redemption capabilities.