Can you find a solution to substitute "Mixin Messenger" with a more well known platform? You say you use it to create the wallet... but will Electrum not do the same thing?
It is about time that someone figure out a more user-friendly method to use Multisig and timelock features. In any way, I will monitor and follow this thread... it has potential to be one of the good solutions in the Bitcoin space.

It just needs to be more decentralized and Open-source for transparency.... preferably without revealing your private telephone number.

Ideally you would be able to use any Web3-enabled wallet supporting, I dunno, a protocol like "WalletConnect", but the problem with this approach is that most of these wallets only work with ETH-like coins.
Its just a consequence of using TIP for authentication, but I believe any other identifier would work with the underlying algorithm besides phone numbers. Same goes for PINs - it could actually be any string of characters, but since it's a mobile app, it's easier to show PINs I guess.