FWIW, my reverse-lit refugium (on 16h/day) is keeping my pH swings between 8.06 at the lowest at 04:00, and ~8.12 peak at ~13:30: my lights come on at 14:30, are on fully at 1600, sunset begins at 22:30, and lights out at 00:00. I test alk every other day at least at 22:15.
Even when I was doing two part with calcium in the morning before lights on, and Alk dosing during the "day" (for the display, that is), my peak was 8.2, with the same bottom end. Now that I'm back to dosing just AFR hourly (starting this last Monday, 13 Apr.) between 00:00 and 08:00, the swing is a mere 0.06 pH. The few times I've done low- or no-lights in the 'fuge, pH wobbled by as much as 0.3 or 0.4 in 24 hours.

(
Started AFR again Monday morning [April 13] at 00:00: once it stabilized 24-hours after cutting out the two-part, it's back to the same ~0.06 pH daily swing that it did in the past.)
Granted, my 15 gallon fuge is nearly 1/3rd of the water volume of my 50g display (so, kinda big by comparison to the small 2 - 5 gallon section in most sumps, especially when considering most sumps with a dedicted fuge section are on tanks much larger than mine), but I'm only pumping about 100gph through it from the display tank. Still, I can 100% credit it for my pH stability.
Admittedly, a similar-sized or smaller fuge on a distinctly larger tank admittedly
might not have as much impact, but if you light it brightly enough, long enough, and keep a big enough mass of macro in there, I'd put dollars to donuts that you'd see a measurable difference.
I've been able to keep my alk in a safe range, but I definitely spent some time tweaking it and fiddling with dosage rates, and it has definitely wobbled up and down some as a result as I've been adjusting lighting and some other things
May not be a practical option for you, depending on setup and situation, but I think a reverse-lit refugium with rock, substrate and macroalgae is well worth it if you can manage it - mine is actually a separate tank on a shelf next-to and slightly higher-than my display, gravity-feeding back into my filter section:
(For those of you who haven't seen it before...)
All of my "ugly phases" in the last year have either been
completely contained in the refugium itself, or, at least >90% of the worst of it stayed in the fuge - the only thing that really ever leaked through (as it were) was FHA: never even had a proper diatom bloom in the display.
As a bonus, that means I also
always have a tank lit with stuff to look at, no matter the time of day, and I can stock it with cryptic/delicate stuff that might not fare as well (or would just disappear) in the larger tank with higher flow and more territorial livestock. This way I get two tanks, without doubling my maintenance.