Bad plan! Sorry, but I gotta be honest.
1) with black algae and a serious bloom, all you'll do is shorten the life of you salt cell by pushing it into boost mode. Instead use bleach or liquid chlorine to raise Free Chlorine level. If your Calcium level is low, you can also use Cal-hypo to chlorinate. Calcium levels are irrelevant in vinyl pools but need to be maintained between 200 and 400ppm in concrete/plaster/tile type pools.
2) with your pH at 7.3 why are you adding acid?
3) The ONLY algaecide we recommend is Polyquat 60%. If the bottle says "Poly...<something unpronounceable and long>.. 60%" and nothing else, that's the stuff. The rest just make things worse. Chlorine is generally the best algaecide.
4) Using chlorine and not your SWCG, you want to raise your Free Chlorine level to 20ppm. Again, you'll just seriously shorten the life of your cell--and they run $400-$600, ball park.
5) "Shock" is a verb, not a noun and when pool stores (like Leslies) use it as one it doesn't tell us much. What's in it is what matters. And Leslies, like many companies, is notorious for NOT saying what's in it: Is it Tri-chlor, Di-Chlor, Cal-Hypo or "Chlorine Free"? So I don't know what it is .
6) That's an awful lot of salt to be adding monthly. Unless you are losing water, salt levels shouldn't change once you hit 3000ppm. Maybe one of our SWCG experts can help.
7) Here's what I think you should do
a) put the SWCG back to normal
b) Add enough chlorine to get your FC to 20. 1 gallon of ultra bleach 8.25% will add about 3ppm of FC...so you need about 5 gallons initially.
c) Check chlorine 2-3x/day and add bleach till you hit 20ppm
d) don't mess with muriatic acid or adding borates now as your pH is fine. It's one of those alligators and draining the swamp things.
e) Brush the pool daily, ESPECIALLY the black algae spots.
f) Vacuum the pool daily, to waste.
g) you should see a change in a few days.

Good Luck!