To be honest, while crypto is probably the closest we have to an actually private payment option (Monero etc), I'm generally not a fan of it for this usage because unlike the rest of the app, it has a much steeper learning curve AND is a common target for scams, which makes it much less approachable for the average user. I love Signal because my mom can use it and I can trust that she's protected, but I would not recommend she tries using the payment option within it regardless of what coin they use because the rigamaroll of going to some exchange to buy it is already a dicey proposition.