I think there may be some confusion re the 90 day thing. This is what I have come up with:
"Every EU citizen has the right to reside in another EU country for a period of up to three months, without any conditions or formalities other than the requirement to hold a valid identity card or passport."
This is the crucial point. You can be in any EU country for up to 90 days without having to jump thru’ any hoops, have any papers other than a p/port - just be there.
Then, depending on the EU country, formalities and an application MAY kick in. In Spain, this is the case. After 90 days, I had to register as res.
FRANCE HAS NEVER HAD THE STIPULATION TO REGISTER AS A RESIDENT UNTIL BREXIT MADE IT NECESSARY BECAUSE ON 31/12/2020 BRITS LOSE THE RIGHT OF FREEDOM OF MOVEMENT.
So, although it would be advisable to get to France tomorrow, under the conditions that apply today, 30/12/2020 would do if you can demonstrate the intention to stay.
My assessment above is based on this:
"The withdrawal agreement concluded between the United Kingdom and the European Union provides for a transition period to run until 31 December 2020. During this period, British citizens and their family members already living in France or wishing to move to France before 31 December 2020 retain their rights as European citizens and are not required to hold a residence permit.
Pursuant to this agreement, these citizens will need to apply for “withdrawal agreement” residence permits. However, they have a long timeframe in which to make their application since they will only be required to hold a residence permit as of 1 July 2021."
So, one does not have to have got one’s feet formally under the FR table by 01/10/2020, by my reading of the above.