The decline of the Borg as a threat was very noticeable in TNG long before Deep Space Nine. Also Data is a great character and does great stuff but his 'brother' just sucks the energy out of the room and I think has a lot of crap storylines.
Its kind of funny how late some of the most iconic things about Klingons were actually revealed given how every later story uses those elements. That's not too unusual in various media but its still something.
I could never watch DS9 because Sisko's way of speaking drove me up the wall but I do have to give props to Star Trek for instead of going "here is the never before mentioned best friend of the main cast who is amazing and has had many many offscreen adventures with them" they took a more subtle "If I beat his traitorous face in with the paperweight on his desk would people believe its a suicide?" approach. It rarely done and worked quite well.
On the Ensign Ro thing, that actually sounds more interesting than just doing a tour of duty elsewhere. I mean it sounds like a hardened officer in the body of child working through some issues for a year would offer some opportunities for comedy, tragedy and awkwardness more than taking a year out to do unseen work stuff. Could even do a tie in episode where they come back and are either now very well adjusted or annoying/worrying the rest of the cast because they've gotten too immature and dependent whilst away and now have all their responsibilities again.
It’s a good point about Rascals being a typically Animated Series plot. They even use the transporter to restore Picard and co back to their proper appearance - something that TAS did in three episodes. Including one where the crew had been youthed.