For what it's worth, I keep everything in a single deck. I never stop reviewing stuff and "interleaving" makes learning more effective than "blocking" (studying things in blocks). Source: Bjork et al 2013 (https://www.researchgate.net/publication/231610455_Self-Regu...)
I took a modified approach to this in university, though nowadays I mostly do the same.
For each new class, I would start a new deck, and after the course was over I would move it over to my main (huge) deck, and it's worked surprisingly well.
Typically (now that I'm out), I add small cards from time to time to my deck, but if there's a large quantity of things I am working on learning at any given time (say I'm going to add 50-100 new cards), I'll put them in their own deck for a little bit.
Ultimately, my repetition does end up being a wide array of subjects though.
This is why tagging is useful. Anything cards I added for United had the tag of the class it was from. You can then do a custom study session with just those tagged cards.
