There’s been a few things to package and run core on mobile before, so that seems like a good start. I imagine they mostly work about as good as it’s gonna get from a backend perspective, then you’d just have to build the frontend? I believe on Android you can run a second process so you can just RPC it. On iOS You might have to embed, but I think that’s doable? If not you could try to go the libbitcoinkernel route though that won’t get you a wallet or P2P or (I think) a mempool.