I agree, and this is something I thought about a lot during the design process. In my experience, just looking at the note (#1 in your example) helps a lot more than no repetitions (which is obvious of course), but it's still a huge improvement compared to my previous flow.
As for repetition, I was thinking of replying to the email with what you think is the answer and letting an LLM decide if you remember correctly. Is that something that sounds effective to you?
The link can literally encode the answer in the URL, which you can just hide the raw url it in the emails’s HTML. (Fox examples, put it as base64 chars in query parameters)
Then you can host a (static) webpage that renders the text. This lets you host any users text w/o an interactive site. No live database, no ops burden, etc.
If you wanted to get fancy (which users of such a product probably would probably want) throw in “success/failure” links so your users can report the results and get changed frequency of spaced repetitions based on their success rate.
Just use CSS to hide/show the answers, or a little bit of JavaScript for just that. Or scrolling, if it is text only. Or links to the answer an http server.
I was thinking of replying to the email with what you think is the answer and letting an LLM decide if you remember correctly. Is that something that sounds effective to you?