REST doesn't care about the size of the set of verbs, only that the set be constant. HTTP as one particular implementation of REST just happens to have nine verbs.

I tried to implement an IMAP server a few years ago, but otherwise I don't know much about it. However, I do recall that Mark Crispin admitted that he wasn't a very good API designer, and that much of the complexity of IMAP was due to it being intertwined with the complexity of MIME. But, I can't find the sources for those claims at the moment.

For normal SPA JavaScript/TypeScript browser-based apps, I like the conventional approach of an HTTP "post" for every API call. You send JSON upstream and you get a JSON response back. Basically it's an Ajax call that gets data back to use to update the page.