It depends on how many cards you’re doing at once - e.g. if you learn 3 N5 vocab a day then doing 2 N5 + Core 4K is the same workload regardless of the fact that it’s an extra deck
I do generally 1 vocab + 1 grammar deck (N4 for both) but, especially now I’m coming to the end of N4 stuff, I’m not too strict on whether what I’m learning is N4, N3 or some random other deck
I keep track of vocab (and occasional grammar) in certain video/tv series and in certain books/manga, which I will occasionally add to my reviews if I’ve seen a word pop up multiple times across either that series or just in general seen a few times and think it’s useful