Well in soil a meter isn't quite as critical.....if you can get by with strips or drops you could do that. But in your case I believe the issue is your very alkaline water. But yes, nothing beats a good quality ph meter that is calibrated often.

Bob, I honestly use the soils I mentioned BECAUSE I don't have to worry about ph issues. ANY plant's ph will lower in older soil after time so you either need to flush or transplant.

As long as you are careful, I personally have transplanted plants 6 weeks into flower (because that's when the ph issues became most apparent) and let the soils do most of the work buffering ph. It's easier to transplant than flush....but not always possible to do. Imo, the age of the plant doesn't matter for transplant as long as you are careful.