Oh, I didn't read your first post closely enough and assumed the Fluval was a skimmer, not a canister filter. I have nothing against them, as my oldest tank uses one, and people like to crap on them but I think they're very efficient and easy to maintain, assuming that you do. Keep the filter running at all times to make sure your bio media stays active and populated. It's a bit of a PitA but clean your filter sponges regularly, ideally with each water change in the early stages (just squeeze them out thoroughly in the saltwater you took out of the tank). When I started my canister system, I added coral too early (mid dinoflagellate stage), but I also didn't run any chemical media (phos remover, carbon), just ceramic rings and sponges, and all export and level maintenance was done via weekly 10 gallon water changes (60 gallons empty system).
The detectable presence of any ammonia or nitrite would suggest that your cycle is incomplete, but when it's near-zero I find tests can be a little misleading. I've never relied solely on water testing to determine when my tanks were ready, I've always used the algae cycle (diatom>cyano>dino) and THEN confirmed with a test once the dinos started to die off and were getting replaced with green hair and/or bubble algae. Some would suggest that PH of 7.9 is a little low, but afaik it's hospitable to marine life, especially if you don't include SPS. Rather than chasing higher ALK, especially since the tank is fairly new, try and maintain the ALK+PH that your freshly made salt water has, which will depend on the salt you use, I think the stability is going to be more beneficial than the "ideal range" this early on. Once the tank is stabilized, then you can start bringing your levels up, but if your salt mixes at a lower ALK than you want to maintain I would suggest considering researching other mixes and when you do decide to switch, bring the tank up to those levels slowly in the week(s) before you use the new mix.
Yes, eight.four (that's it's actual name, the words instead of numbers) is a bicarbonate+carbonate alkalinity and PH additive. I haven't heard of people using baking soda (sodium bicarbonate) in saltwater, only soda ash (sodium carbonate), but that doesn't mean anything other than a suggestion to compare/contrast the dosing and effects if you haven't already. And no, I don't see the duncans and frogspawn being the main contributors to your ALK drop.
Side note, if you notice a drop in water movement as your pump heads get encrusted with coralline, you can get food grade citric acid cheap at Bulk Barrel/Bulk Barn. Less acidic than white vinegar, so it should be a little kinder to your gear, but it does the job of eating away at coralline and doesn't stink like all hell, and less of an issue if you happen to spill it like I do haha