IMO, I would stick with the Jack’s 321 and no RO system. Try that before adding more stuff. I don’t see anything with your plants that screens change something.
A lot of folks here have a little nute burn from using Jacks. I don’t even worry about it anymore. It never gets worse. Jacks really is so easy. Mix and feed exactly like they say, full strength, seed to harvest.
The damage that I see on your plant could have been when it was younger. When you have an issue, even though you catch it early, the damage is almost always already done. It just may take a min to show up and when it does, it may continue to get worse, and may not.
The pH buffer might be something you need if you can’t get pH stabilized.
pH is a funny thing. It will want to revert back to a neutral state. 7.0 But, after it’s a certain value with Jacks, provided conditions are close to optimal, it will remain stable… unless you see it won’t. Then I would try to buffer.
I use Armor Si in mine which raises the pH. I have 22 gal in my system and have to use 3 tbs to get my pH down to where it will hold it. I use to do it a little at a time and would always have to readjust. I have learned how much it takes in my system to keep it stable.
This is the part that has to be done to get a base line so you’ll have a better idea of how everything is mixing and how long it takes to get where you want to be.
I would mix a gal of Jacks.
Check pH and ppm’s. (document)
Let it sit for about an hour recheck and document. (stir periodically throughout that hour.)
Next I would adjust pH to where you want it.
Let it sit for about an hour. Recheck and adjust as needed
Repete till you have the pH where you want it.
Then add up the total amount of pH down used to get you to pH range. That will be close to what it takes to get your pH stable in 1 gal of nutes.
This is if your running a hydro system, all water, or mixing your water up in bulk. Jacks will last a good while just in water as long as it doesn’t get too warm.
Hope that made some sense.