When you do decide to get a PH meter, the better ones have a two-point calibration and a glass bulb as a rough rule (they also start at twice the price of the cheapest ones). PH calibration needs to know where 7.0 is (which is a one-point calibration), and also where 4.0 is as well (the second point) so it can figure out the slope that the device currently has so it can tell everywhere in between 4.0 and 7.0 far more accurately.
Some of the cheaper ones are no better than looking at the liquid, some are worse because the liquid will always tell you the same, correct, result instead of bouncing around 5 different incorrect values. If you can, get the testing solution that comes with a little card instead of looking at your phone, the screen is backlit unlike the tube and this can make it hard to check one colour against the other.
Just under 6 is a good place to aim for. It gets you most of the things you need except zinc, which is more free at 5.5 (but not needed as much as the chemicals you get at 5.9)