There's https://lingonaut.app/ , which is not open-source but at least open data. It's sort of like early-days duolingo with volunteer-driven courses. Some of the courses are surprisingly well thought out (if yet to be implemented to a usable extent), but quality will be mixed. I don't think they have an answer for proper pacing at the moment, but being a free product they're at least not incentivized to keep you repeating the same content indefinitely.

I'm getting a lot out of Anki with premade decks these days, combined with watching tons of video content.