Was in the same situation as you. Applied for a visitor extension in Feb. My IEC ran out in mid April.
Visitor extension took forever and I eventually just went ahead and applied in early June. Visitor extension came two weeks later.
I specifically mentioned in the visitor extension app that I planned to apply for PR.
Would also definitely advise you to wait those two extra weeks but no need to wait for the extension to come (especially if you are from a visa exempt country).
Edit: Just to elaborate why waiting those two weeks is a good idea. The officer needs to be convinced that you actually lived together for one year continuously. If you submit the very day of becoming common-law they will consider anything before the date of submission.
For that period you would be two weeks short of cohabitation proof. Even though the officer full well knows that you definitely became common-law by the time the application reaches their desk, nothing past the submission date is allowed to be considered for the establishment of the common law union.
Therefore unless they cut you some major slack they would almost have to reject your application.
I, for instance, would not have applied before late April, when I received some letters in the mail under our shared address, even though we became common-law in mid-April. There was no lease either since my partner owns the home.