You can apply WP extension 3-4 months early. However, once you receive ITA it takes upto 6 months to get PR approval. The void your extension if they see PR is approved. So don't worry about it and go for Extension in 2-3 months while you are still waiting for ITA.
I read the comments on previous posts, it seems the trend is one has to wait at least a month time after Express entry profile creation date in order to receive IOA. I guess very less number of nominations are left for 2019 ( 6000 + nominations would have already gone by now). Hopefully I will be lucky, lol, fingers crossed !