OK, same problem in the other direction. But messy increasing the RX buffer size on hardware serial and the Nano is memory limited so
suggest you try this
On the ESP8266 side, process the json there and send only the two numbers comma separated. They will fit in a standard 64byte RX buffer.
replace SerialComs coms(...) with just SerialComs com;
That sets up a 60byte message size both ways
Here are a couple of sketches using comma separated values. You will need to add the Json decoding in the ESP code.
NanoCSV.ino (1.8 KB)
ESPtoNanoCSV.ino (2.3 KB)