Do NOT bring your water pH to 5.5-6.5! The correct pH for potting soils is 6.5-6.9, and soilless 6.3-6.6.
Looks like low soil pH causing lockout and rootbound causing crappy soil chem.
Is that a pot with a built-in catch tray?
Get it off and check your runoff pH. Also slide the plant out when the root ball is dry and look at the roots.
I foliar feed with GH Micro while I am trying to solve these sorts of things, to bypass the roots and allow feeding to continue while the problem is sorted.

You may be ready for a re-pot. Either way, I'd do it- your soil chem is off and needs fixin'. Flush @ 6.9, feed lightly @ 6.9, allow to dry, re-pot into fresh light soil in a slightly larger pot.