Wireless Artnet Node
I think the ESP32 is powerful enough to do both WiFi and DMX. Splitting between two micros won't help you much as you'll still need some communicaion link between them.

This guy is doing it with an ESP8266, which is much less powerful: https://github.com/claudeheintz/LXDMXWiFi_Library

On the mesh network side, you probably want to look at this https://github.com/espressif/esp-mdf , rather than running as a WiFi access point.


  Also how does the Serial buffer size affect the ability to output the information of a 512 channel universe, would a 256byte buffer be too small?


This is a difficult question to answer without knowing the data rate of DMX and the CPU loading required to do the mesh networking, but the answer is very likely to be "256B is ample".