If you enjoyed the OP, then Andy Matuschak’s article on how to write good prompts (cards) should also be interesting for you. And if you want to learn more about the research behind spaced repetition, then Gwern’s article is pretty good.
20 rules for formulating knowledge [1] are also very useful. They were written by the creator of SuperMemo, which to my knowledge developed a lot of the spaced repetition algorithms.
For example Anki, the software talked about in the article, uses the SM-2 algorithm by default which was developed by SuperMemo.
While SuperMemo is at SM-18 in meantime and also much ahead of Anki in other features like incremental reading (which in my experience is its real super power, not the spaced repetition as such). Although there might be plugins for Anki that offer something similar.
That's one of my major gripes with Anki, it's just a bit too brittle for my taste. SuperMemo is not without its quirks (I mean, it's a quirky niche software to begin with), but all in all, I never had any major problem with it and I've been using it for more than a decade now.
I think for most people who want to get started with SRS, Anki is just fine. A lot of people like to hate its UI but I personally find it perfectly fine (and even good before the recent update where they forced drop shadows on random UI elements). It being available on every major platform is also a major plus point for when one needs to review cards with no access to their workstation.
That being said, the $40 is probably what holds most people back from using SuperMemo. If I had $40 to spend on SRS without feeling guilty I'd 100% use SM over Anki.
For example Anki, the software talked about in the article, uses the SM-2 algorithm by default which was developed by SuperMemo.
[1]: https://www.supermemo.com/en/blog/twenty-rules-of-formulatin...