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For flashcards/spaced repetition I've used Anki quite a bit.
https://github.com/ankitects/anki
I did get good enough at Chinese for a 3-week trip but I only needed to order food and navigate streets.
I have no idea how good it is for "fringe" languages.
As a language professor, I can tell you that 2-hours a week will only maintain your level of a language. Ideally, setting aside time every day is the most important thing.
I have looked at Anki before and I am familiar with it. I have a few decks on there, one for German via Nicos Weg and another for Japanese Kanji Radicals via Wanikani (I think that's what it's called). I suppose re "fringe" languages, it's only as good as the decks, and there might be some out there for them.
Anyway, I feel like Anki alone isn't enough for me. I can sometimes get a little demotivated to study for a day or two and then I'm set back in my studies (or at least I feel that way).
There are a couple of language learning tools I know of that are FOSS, the first is of course anki, which you know of, the other two are both focused around reading, specifically Lingq like tools:
If you're not familiar with how Lingq (and the above two lingq-inspired tools), you provide it content and then it highlights unknown words, you can click the words which show you what they mean and you can then add it to a set of learning words. Over time through these words coming up naturally, you learn them. I've found it just pretty much lets you more easily read interesting texts earlier, because looking up words is far easier.
I will definitely be considering LWT when I look for apps to self-host once I move out of my current place (long story). Might even look into developing a better UI if I get the time.
I have neither used a dedicated app specifically for learning the language, nor completed "learning" the language to a passable level, but I did start learning japanese using only freely available online resources. There was quite a lot of information in video and text form regarding grammar etc.. For training vocabulary and the japanese characters (I only learnt Hiragana and Katakana - no Kanji) I used Anki. In my oppinion it's very doable to learn a language this way without a course (though keep in mind that your pronunciation will suck if you don't get feedback from people that actually speak the language) - in the end I was just too lazy to learn vocab.