Cloudflare is a nice option if you can adapt your system to run there [1]. You could also run just the frontend on CF and host any APIs elsewhere. Or check these sites [2] [3] with some good reviews and comparisons of hosting providers.
Regarding spaced repetition, RemNote 1.17 an 1.18 has a bunch of AI features for summarizing PDFs and YouTube transcripts and turning that into Q&A for spaced repetition, and they also have a Chatbot interface in-app. Perhaps you could check some of what they are doing for inspiration [4].
RemNote looks really cool, and it seems like they are exploring similar problems to the one's I'm trying to solve. It will definitely be a valuable source of inspiration. Thanks for putting this on my radar!