Hello again, Ben.
I would trust the cheap pH meter, as the reading is much closer to the reference solution (assuming that's
accurately measured!) You might purchase a pH reference solution from a reputable third party to check.
It looks like your RO/remineralization system is operating at the lower end of the hardness range, 30 ppm plus 20 ppm, so you're getting about 50 ppm, which is a little low for optimum extraction without scaling. Assuming the cheap meter is correct, a pH of 6.8 isn't cause for great concern. I seem to remember reading somewhere that 6.8 is the minimum recommended by LM. I would be concerned about a pH of 6.5, however.
Also, 45 ppm is a bit light for optimum extraction, at least according to the experts. As I recall, Jim Schulman recommends something like 70 ppm, which should get the pH at neutral or above. (Personally, I think coffee water chemistry more complicated, given that I get excellent extractions with 0 ppm hardness and 140 ppm alkalinity from my cation softener.)
I see no problem with adding a little bicarbonate to bring up the TDS and pH. I think bicarbonate of soda is acceptable, but most coffee water recipes I've seen use other chemicals. Here's an example recipe from George Howell
that I've made. It uses potassium bicarbonate and magnesium sulfate, both of which are available from Amazon. You could add a pinch at a time of one or both of those until you get the desired TDS and pH (measuring each pinch so you can repeat the recipe.) Plenty of other coffee water recipes here on HB and on the web.