I've never been a good assessor of plant deficiency symptoms. When a plant gets "bad" in my eyes, I usually go back to the light and CO2 first, then make sure the macros are ok, all the while maintaining micro dosing. However, I haven't had a plant problem in years. So, a couple of thoughts for you to check out as an option to viewing your issue from the standpoint of whether or not your Mg and Ca are good.
As you know, when you test GH, you can't tell what the ratio of Mg and Ca are - could be all calcium (not likely). So, like you, I wanted to know and I have a FW tank. I found what may be the only FW Mg test kit in the World (from what I can tell) that seems to do a decent job of testing Mg only (I calibrated by making my own known mixes of Mg and Ca). It's made by JBL (JBL | Vorsprung durch Forschung
) in Germany. I got it via Ebay from one of several British sellers. Make sure you get the FW version and not the SW version. I forget what I paid, but it was reasonable at 2-3 times an API-type kit.
Along the lines of what Seattle_Aquarist was mentioning, also look up "Redox Potential." It speaks, in general, about the ionic charge and a big takeaway is that GH can remain high, but the beneficial charge may be gone. Essentially, you need to revitalize the GH with new Mg and Ca additions occasionally. UV sterilizers also help tremendously with redox. They do other amazing things, as well, and I wouldn't be without one, but it needs to a good one like the Vecton brand. Cheap Amazon types are only good for clearing up a little green water.
Last, if desperate, you could go out and buy distilled water (since you don't have an RO unit) for every water change and add the correct amount of Mg/Ca. Then see if the problem persists over a month or so. That way, you might be able to rule in/out your tap water concern. A lot of people use GH Booster, but I prefer Seachems' "Mineralize" (in the aquavitro line - LFS only) because it is Mg and Ca ONLY. All other boosters include potassium.