The cow doesn't have to leave the barn. There's plenty of time to correct for sub-optimal pH. Collect your mash sample at ~15 min into the mash, and cool quickly. You don't need much. All you need is an amount of liquid/mash that covers the bottom of the meter's glass bulb. Give the mash a stir, collect ~50-100 ml in a beaker, swirl it in an ice bath for ~30 sec, and it's good to go. If you need to add acid, there's still plenty of time to do so to get that optimal pH.
Back to meters: I too bought, with much regret, the Thermoworks 8689. It never worked out of the box. I contacted Thermoworks about it, worked with their support people to try to get it to work, but it never did. Whenever I tried to calibrate it, the pH would jump all over the place--literally it would span 2 or 3 pH units (not 0.2 or 0.3, but 2.0 or 3.0!) and never stop jumping. I followed the instructions to the letter, it never worked right. When I asked if I could return it, they said no, since I had opened the box and used it. Cool, thanks. I needed a paperweight anyway. So I ordered the Milwaukee pH56 pen and it works great. No complaints about this one. Highly recommend it. There may be more accurate meters out there, but for me, ballpark is fine, and for the price, this meter nails it.