You're not going to beat out Venmo, Apple Pay, and other established platforms.

Those are established platforms where you are. Anecdotally, no-one here (NL) uses Venmo or apple pay, recently there's been an increase in popularity of 'tikkie' (https://tikkie.me/) which allows people to send payment requests through whatsapp (used by 90%< of people here). so if something like this is ingrained in whatsapp directly apps like that would lose popularity, i think