Generally I only create new cards when I run into a verb form in the wild (as it appears in whatever example sentence I run into). If I'm having trouble with a particular conjugation, then I'll get example sentences for every conjugation form of a particular verb (different sentences for each form), and put those into my flashcards, but once I've done that, then I usually won't need to do that again for verbs that follow that particular pattern. So if conjugating Schlafen is difficult, you might use a bunch of sentences for that verb, but afterwards, you wouldn't do the same thing for, say, Braten.

You may prefer what I've been doing lately with Spanish:

Here's "hice" (I did), the past tense first person form of "hacer" (to do):

Card 1:

(answer "Hice.  Root/Base Form: hacer")

Card 2:

Just says "hice". Back side has example sentence.

Card 3: 

Says "hacer". Back side has example sentence for "hice" and says that "hacer" is the root/base form of "hice".

Card 4:

Answer: "hacer"

If you're plateauing, try playing around with this: https://fluent-forever.com/hacking-fluent-forever/ - do one or two hours of that, try to focus on talking about stuff that's difficult to talk about, and see whether you develop a lot of content that works specifically around your weak spots.

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