Im kinda confused about how you flush. Flushing with a ph of 7.5 with a soil ph of 5.9 should brining it somewhere in the range of mid 6's. Also, whenever I flush i use a tiny amount of ferts to replace what I flush out, instead of adding more on top later.

Either way tho, 5.9 isnt really danger level.. I mean its on the low side for soil but not dangerous like 7.5 is for the high side. I would do a serious flush with some light nuted water around ph 7 (you can get aquarium ph upper, its easy to find) if you're really worried about it tho. Are you having any lock out issues?