I can afford to run a small VPS, and even a small VPS nowadays is many times more powerful than what is required to service a simple application to tens of thousands of clients, with some care. It might be fun to just treat that as a sunk cost and build something that's fun and easy for folks to interact with.
These 'shower thoughts' are brought to you courtesy of me reading this extremely fun post about One Million Checkboxes: https://eieio.games/essays/the-secret-in-one-million-checkboxes/