Mine failed, apparently (according to the IO) because I had recently come on to a non-imm retirement visa. I had to do this one and the next in person, then I can do it online.
It was easy to register and submit. Give it a try, and keep an eye on your email for a rejection or approved email
If you are changing from a tourist visa or visa-exempt to a covid extension, it goes from the day you apply.
If you are getting a further covid extension on top of another one, it gets added to your stamp.
Depending on the office you use they will let you apply once you have less than 30 days remaining. However some are less than 15, 7 and 3 days respectively. So it depends on the office you use, and how busy they are.
Also bare in mind that you'll likely get an under consideration stamp first and then have to return two, three or four weeks later, again, dependent on your office.
My advice is (predictably) to speak with your local immigration office and ask what they would offer and how it would play out.
if I were you, I would find a way to go before the last day. Either tomorrow or Wednesday, in case there is anything else you need to get the stamp. Otherwise you'll be pile of pain and need to all start over again. Best of luck