I'm not paid enough to generate primes for RSA by hand :P

This gives me RSI

Although Solitaire is an impressive literary device, since you could in fact build this and it is in fact good enough to serve the purpose it serves in the novel...

It's a symmetric algorithm, so this doesn't solve any problems Monzo actually has, whereas the elliptic curve public key crypto they use does

It's known to be cryptographically weak. If you write small messages, by hand, and your recipients are, even if they have more technology, reluctant to try to decrypt large volumes of messages too great in number to all be from you, you're safe. But if you used this industrially it would get broken at scale.

As a comparison, RC4 has a biased keystream just like Solitaire, and it has been successfully attacked in practice since unlike Solitaire it was actually in use on the public Internet. The attack consumes a very large number of messages, transmitted over HTTPS in the course of hours and so would not be at all practical for a system used with a few hand-ciphered messages.