You should have know that as it's common knowledge.
To be fair at one point some advice was 15 days prior up to due date.
As for the rest of your post ....Yes you apply as soon as your window opens is best policy.
So that means at the 75 day mark.
The report starts clock for next report.
You would end up doing one extra report doing it online vs in person per year.
If you do report via mail you can also do it at the 75 day mark however immigration with give you a due date for next report 90 days from previous due date.
Not from when you mail it.
As for your extension.... people moan about that for what. Best to provide what immigration demand.