pH of 6.5 is not too high for soil, that is near ideal, you can go slightly lower, maybe as low as 6.2 in soil.
The fertility meter, the numbers mean nothing. But it apparently is having a problem reading all your nutes, and your nitrogen is probably really high without the others being high, to have your probe read that the fertility could be maybe low but still obviously burning your leaves with too much nitrogen.
A flush will still lower the nutes, including the nitrogen. This is exactly why I don’t like using most miracle grow products on cannabis, they often have way too high of nitrogen and maybe not enough of the others to keep your plant healthy. I would never use miracle grow products except maybe the miracle grow for tomatoes or maybe the drops for cacti, and then one should follow the directions on the product as to recommended feeding.
I would also not use most of the stuff in the list on the link, and I don’t necessarily think Latewood would either, at least not in a small indoor grow, most of that stuff probably doesn’t work fast enough or reliably predictable enough, from what i understand, but I’m not him, and so I can’t say for sure. That is what I mean the he will have to say for himself. Also OldGoat has used some of that stuff, so he can also chime in with his experience.
As for normally lowering your pH in soil for cannabis, you can flush with water that is at a pH of 5.5 and keep flushing until it comes to the pH you want. I’ll invite Latewood to this conversation so he notices it. I’ll invite OldGoat also, so he can chime in.
Oh, also where are you sticking the probe, the top of the soil might not read at all the same as deeper down, near where most of the roots should be, the “root zone”. Try and get a reading from the bottom lower half of the soil, maybe a reading through a drain hole on the side of the container?