Right about the time that TNG ended and those writers were freed-up to move to DS9, if memory serves.
No. I think you're confusing the "finding its stride" with the main plot arc starting. DS9 found its stride either late in S1 or early in S2, depending, which is before TNG finished.
The writing quality doesn't notably increase from S2 to S3, either, I would suggest. Also AFAIK, there was no exodus of writers from TNG to DS9, though perhaps you could say who if there was.
And the Dominion War arc was planned before TNG finished too (and started to be implemented).
en.wikipedia.org
What did happen around that time was that Piller, who together with Behr, had created DS9, move off on to VOY. I haven't got all the details to hand, but it seems to me like more writers moved from TNG to VOY, than TNG to DS9. I can look into it further if needed. So if we see a change it's that Behr runs things better than Piller (though is possibly more of a dick to the cast, I don't recall exactly).
DS9 had some highly critical reactions both before and during its first season from fans who hated the idea of a fixed space station instead of a starship. Even back in the day they were vocal, on social media they'd have been drowning out conversations.
That's a really off assumption.
Those fans were loud, but they were
wildly outnumbered. I was on the internet back then - they were mostly the same people who hated TNG, too, and who were still saying "TOS was better". They would have been crushed on social media because they were much older - like 20+ years older than people who were open to DS9, and thus would have been way worse and way slower at posting and memeing.
I've still got fanzines from back then (I was 15 in 1993 - some of them are near-professional quality too), and DS9 had plenty of people excited about it - way more than VOY did a couple of years later.