I learned a significant fraction of my english vocabulary by watching all of the new Doctor Who show with subtitles (almost a decade ago). Your mileage might vary.

If all you want is be able to read, I don’t think translation flash cards are the way to go (these are useful if you want to to be able to quickly find a corresponding word in your native language). When learning to read in foreign languages, I create flash cards where the “question” is a sentence in the target language (German in your case) with one unfamiliar word, which I put in bold. I succeed if I correctly understand the word in the context of this sentence. In the answer, I put the definition in the target language (German here) for the appropriate sense.

Usually, this is enough for me to understand the word when I meet it in new sentences, assuming it’s used in the same sense. In cases where I failed to learn it, I just add a second flashcard with a new sentence containing that word.

For “encyclopaedic” words like species of animals or plants, kinds of food, towns, etc. it’s in fact more useful to put a translation in your language rather than a definition in the target language, assuming it is a familiar concept in your own language. (And not try to learn several similar animals, plants, … at once, otherwise I mix them up.)

I usually take the sentence from a dictionary targeted at learners (sometimes I use the sentence where I found the word, if it’s the only unfamiliar word in it). For English, I use The Cambridge Learner’s Dictionary. For German, Langenscheidt Großwörterbuch Deutsch als Fremdsprache might be a good one (what’s important is that there are example sentences).

1. Obviously there are many general techniques for memorization you can use, which mostly amount to moving the task into either sensory/visual or spatial memory. Visual and spatial memory are huge, fat larger than verbal memory.

2. With Anki specifically:

2.1 Include an example of use in a sentence (as a separate note from the bare word).

2.2 I find it is very very useful to bring words in initially only a few at a time e.g. 5 at a time. If I bring in 50 new words I find, as with your experience, I am cycling around and the cycle time exceeds my memory.

2.3 Do it every day. I found my progress more than doubled

With these techniques I learned 1500 Italian words pretty fast. (the vocab required for B1 level).

3. Reading really helps to build vocabulary. You get exposed to the most common words more frequently, in an automatic and natural way. Start with really simple material and build up.