For batches that size a pH meter is not a luxury. Even a $150 Hach Pocket Pro+ will do that job. You definitely need to calculate and adjust mash pH. Then measure it to confirm your parameters are as estimated, then fine tune from there.
You may need a 3-roller mill to reduce husk shredding while keeping efficiency high.
I agree OP needs to look at mash pH. After tuning in my brew'n water spreadsheet, I really don't know that you need a Ph meter though. I agree at this scale it isn't a bad call, but not a requirement IMO.
I have a meter and as I eluded to, I found my pH was always right on my what my spreadsheet estimated. I still check it, but the last few batches it has really just been a confirmation activity, I'm quite confident of what it will read before I take the reading.
To get there, every other aspect of my game needed to be precise.
1)Grains entered into sheet with accuracy to the tenth when weighed out.
2) Measuring water additions on a scale thay measures grams to the hundredth.
3) Measuring acid additions in ml via syringe.
4) Accurately measuring strike and sparge water to the tenth of a gallon.
If I nail those physically and on the sheet, my pH is exactly what the sheet said it would be.