Just a thought to share.
Sometimes less is more. Especially at this early stage. Stability is always more important than numbers and any changes should be gradual.
Based on what’s in your tank, I honestly wouldn’t even be checking alkalinity. I never ever checked it when I had fish only tanks. It is desperately important along with calcium for coral growth but I wouldn’t be concerned about a low reading now. Water changes over time will balance it out.
Once you are ready to put corals in, then figure out how to keep alkalinity stable.
I personally use 2 part b ionic and have had great results.
Hope everything is fine in your tank (I anticipate it should be) and just wanted to throw this out there.