Consider that for your use-case the state is likely to be small enough that you could simply serialize the state and base64 encode it, and add it to the url. O

Consider that for your use-case the state is likely to be small enough that you could simply serialize the state and base64 encode it, and add it to the url. Other than that, I'd self-host it, either on a cheap VPS or just a rasperry pi in the closet, this way you avoid thinking about all the cloud stuff and won't be surprised by some massive bill because something went wrong, writing a backend in nodejs will feel very familiar to you if you've already used javascript or typescript before. For storing the state, it depends how much data you expect.. I'd see how far I could go with just keeping an object in memory and serializing that to a text-file.. or you could use sqlite with nodejs, then you don't need to administer a separate piece of software, it can simply be a nodejs module.