I just had a look at the dates, here's what I have.
First post by /cbts/ BO (We can assume the board was created moments before this):
Nov 19 2017 11:21:27 (EST) - https://8ch.net/cbts/res/1.html
Qs first known post on 8ch (#230) was on /pol/:
Nov 25 2017 13:54:45 (EST)
Qs first known post on /cbts/ board (#234):
Nov 29 2017 14:21:16 (EST) - https://8ch.net/cbts/res/10820.html#10925
First post by /thestorm/ BO (Again, assuming board was created moments before this):
Dec 14 2017 22:05:18 (EST) - https://8ch.net/thestorm/res/1.html
Qs first known post on /thestorm/ board (#467):
Jan 5 2018 22:51:45 (EST) - https://8ch.net//thestorm/res/3995.html#4657
Qs first post on /greatawakening/ board (#512):
Jan 8 2018 22:03:27 (EST)
First post by me on /qresearch/ (I created the board just a few minutes before posting):
Jan 8 2018 23:50:58 (EST) - https://8ch.net//qresearch/res/1.html
Qs first post on /qresearch/ board (#516):
Jan 8 2018 23:55:15 (EST) - https://8ch.net//qresearch/res/1.html#7
Qs first post on /patriotsfight/ board (#1311):
May 4 2018 14:59:49 (EST)
{Edit: Added links to posts}