I can't answer your specific question but I have a comment. The maximum effective focal length of the RX10II is 200 mm while that of the RX10III is 600mm, just this huge difference in reach should make a big difference in bird photography. There is a very good thread on the RX10III as abirding camera here in Birdforum.
"There is something fascinating about science. One gets such wholesale returns of conjecture out of such trifling investment of facts."
- Mark Twain in "Life on the Mississippi."