Thanks. So, For now, I will use Alby webhooks, with option for users to add their own webhook API. That's the only API it uses. 

One more thing, Should I use preimage encryption/decryption to eliminate the need for a centralized encryption key & login? 🤔

https://guides.getalby.com/developer-guide/v/alby-wallet-api/reference/api-reference/webhook-endpoints 

 imo LUD21 or maybe even better just relying on NWC (Which has the lookup invoice function or also notifications https://github.com/nostr-protocol/nips/pull/1164 )