I also added Flourish root tabs recently; I can't rule out leaching into the water column but see nothing obvious.
Seachem root tabs contain calcium chloride and magnesium chloride. Both can be found in many GH boosters and dissolve very rapidl in water. The root tabs will definitely increase GH. The question is how much.
If, after doing all of this, you still have the issue, then I'd use the acidic water test in a glass for each item in your tank (including some substrate). I'd probably start the pool filter sand test right away.
Yes I would also do this but you would have to rinse off each item fist with Distilled water to remove the GH and KH you are adding with the root tabs and salty shrimp. then run the acidic water test. I doubt the pool filter sand is the source but test it anyway.
There are all types of substrate that can reduce hardness, not touch it, or raise it persistently. What type do you have?
There are substrates that reduce KH but I know of no substrates that reduce GH. Since you are using distilled water, salty Shrimp, Flourish, and sachem root tabs, most of your KH comes form potassium or sodium bicarbonate. The GH test will not detect sodium or potassium. It will only detect calcium and magnesium. So the only way to reduce your GH is to find the source of your excess calcium or magnesium and elliminating it. Your substrate should be mostly silicon dioxide, aluminum oxide and iron oxide and should be inert with minimal effect on GH, KH, and PH.
Note I never had much success with flourish comprehensive in my RO water aquarium. Algae was a constant problem with it. It depends on your water source having some micro elements in it. But your distilled water is very close to pure water. Which can easily cause a nutrient deficiency which can cause algae problems.
I would recommend considering switching to Nilocg.com Thrive+ fertilizer. I think it would work better in your aquarium with distilled water They also sell ThriveS but I wouldn't recommend it sine it is missing Copper which plants and shrimp need. With NO copper your plants would likely not have enough to grow. For your tank with distilled water make sure the fertilizer you use has copper in it. Yes too much copper can kill shrimp. But I have not found any fertilizers with a dangerous amount of copper in it. So I wouldn't worry about copper.