Definitely overthinking medium ... HF out of bag floats around 6.3 ( ish ) ph out of bag ....
All you have to do is match the medium as far a watering and feed mixes - 6.5
Dolo needs time to break down and under most circumstances with HF or FFOF the prebuffer gets in soil already is enough.
EVERYTIME you run a runoff amount of water through it , you leach / unbalance it. The soil needs very little.
FF nute schedules should be taken with a grain of salt as those amounts listed can be too much and it is better suited only as a guide .
FF nute lines will salt up fast with heavy excess. Starting at 1/2 strength is a good base line , also not every bottled nute listed is necessary. The brown spots are from ph swings , which continue as you continue to flood medium.
Since you are in bloom , run a 1:1 mix of ( 1 tsp of Gro big and tiger bloom ) at 6.5.
Small bump to nitro , let plant settle in on ph and then run tiger . Also I would add a scoop or two of fresh HF on top soil and water in when container goes dry. Try to rebalance the medium and let it do the work.