Maybe a SF ship could sustain 9.9 if it were always in full repair?
The USS Prometheus was able to sustain 9.9 without problems... so I don't think a state of 'full repair' has much of a merit... well, it obviously does because you can't be expected to haul a half vaporized hull at 21 473 times speed of light without reinforcing it with forcefields etc.
My guess is that since we hadn't seen Warp speeds for SF ships exceeding 9.6 (except in only emergencies) in TNG and DS9, Voyager by same admission would be limited to a TEMPORARILY sustained cruising velocity of Warp 9.75 (no more than 12 hours perhaps)... it was just never designed to do Warp 9.9 for any period of time... at that point, structural collapse would occur in under 1 minute as we saw in Threshold (which was arguably in line with previous series in terms of Warp capabilities of SF ships of that time).
So, I'd imagine that starting with the Prometheus, SF found a way to shore up the Structural integrity field (SIF) without draining too much power (aka vastly improved efficiency) and had more powerful warp cores... but the Prometheus also had the benefit of multiple Warp cores at its disposal if I'm not mistaken, so that's kinda like brute forcing your way to sustainable Warp 9.9.
However, that was about 3 years after VOY was lost... so by the time VOY returned (2378), SF would have probably been able to increase most fleet ships speeds to a sustainable Warp 9.6 or 9.7 (the newer ships like the Galaxy, Sovereign and Intrepid classes probably were already able to reach 9.8 and sustain it without much trouble).
Warp 9.9 would certainly be viable with more breathroughs in efficiency which would allow shoring up SIF and stronger Warp core energy output on recently commissioned ships (I'd say any design from TNG era onward would be viable for Warp 9.9 and maybe a notch beyond).
Older ships might have somewhat limited Warp, but they would receive upgrades with the rest of the fleet on a regular basis, so, reaching and maintaining 9.9 would also be possible for them too (although it may take a year or two longer compared to newest ships - and it also depends on when ships actually go for their overhauls - we've seen the USS Cerritos was a fairly weak ship in Season 1 that faired badly against Pakled ships... after it received a refit by the end of Season 1, it was able to keep up with the Pakled warship and a Klingon BoP for a much longer time without falling apart).
I'd say its a matter of upgrades/refit... most ships are deployed for LONG periods of time... and some might not see a drydock for YEARS... and when they do, its possible they are a year of 6 months early (aka before the better engine upgrades for their ship class are available).
So, by 2383, its possible that most SF ships are capable of reaching and maintaining Warp 9.9 without much effort... certainly enough time has passed (and technology was brought back, studied and probably developed) to allow for that to happen.
It's also not surprising the REV-12 might be capable of sustained Warp 9.9 (its a rather massive ship, therefore it has more space for engines, power core, etc.)... effectively brute forcing its way to that speed like the USS Prometheus may have had to (but the Prometheus was a lot smaller, and therefore was probably more EFFICIENT by design).