The 28-day concession is NOT about when you apply. I repeat again, it is NOT about when you apply. It determines the
earliest date you can be given ILR. Simply speaking, you can get ILR WITHOUT completing the 5 years. This should be very simple to understand. I made this abundantly clear and as you can see below I clearly said this under that link but I am not sure why this is still not being understood:
What is the 28-day concession ?
The 28-day concession means you will be eligible for ILR (i.e. ILR can be granted) within 28 days of completing your qualifying period.
In a nutshell, this means you can get ILR without fully completing your full qualifying period.
This is a procedural concession offered by UKVI that applies to all ILR applications. It is not part of the immigration rules.
You seem to think that you need full 5 years for ILR. You do not need 5 years

That is the whole point of 28-day concession
The
earliest to be granted ILR in this case is 12 December. So the advice is that if you apply for a super priority decision then the caseworker will
DECIDE your ILR on the same day, hence the
date of ILR decision and the date of your biometrics will end up being the same. That is why you often hear to ensure to book the biometrics
on or after this cutoff date if you intend to use priority services.
So when to apply then is NOT really important. What matters is the date of ILR decision (i.e. when the caseworker will make a decision on your application). That date should be on or after 12 December
Advice is given based on my personal research and experience only. Do NOT contact me via private message for immigration advice