If you want to watch more ToS, you can similarly cherry-pick your watch list there, as there are some stand-out episodes. I’d definitely recommend at least watching The Menagerie for context before SNW (or Captain Pike’s appearance in Discovery).

DS9 is some of the very best of Trek, but you’re going to want to actually watch it through; it’s got much larger story arcs and character development than ToS, TNG, and Voyager. It takes a couple of seasons to really pick up, but is well worth it. If you’re not ready to sit through that many subsequent episodes yet, you can wait until you have a better feel for the series. But don’t sleep on this one, it’s fantastic.

Those are my highlights for earlier Trek, but you can’t really go wrong as long as you’re enjoying what you’re watching!