Dunno if watering with ph 8 water is a good idea even if runoff is low. if the plants weren't showing symptoms of pH issues when the runoff pH was low then just leave it imo. To be honest I don't even bother checking my runoff unless I see problems and I water with quite low pH. If your runoff is 6 or 6.5 then definitely don't water with ph 8 actually cos 6 is fine. I read somewhere that you should pH the water with pH adjuster etc but the runoff pH should be adjusted with soil amendments and if that's the case then it could be the high watering pH that is the problem.
You should start flushing about two weeks before harvest but I can't tell you exactly when that'll be, you just have to make an assessment based on the trichome development and growth you've seen so far. If you're using organic nutes then you don't really need to flush imo, but just giving them water for the last few waterings can't hurt and would be the safe bet.
I'm sure other more experienced growers will chime in and give some more reliable advise sorry I can't be of more help.
All the best
TC