I recommend you read the page from which I copied the first para. That para says it all, really. It’s very straightforward.
In my view the FR are being very fair and open-handed about Brits and residency. The FR attitude is that the EU Freedom of Movement Laws are paramount. The FR are content to allow Brits to exercise FoM under the EU FoM legislation, without any additional purely French ifs or buts.
Having exercised that right to the satisfation of the FR - being on FR soil [tho’ see below] and being seen to be intending to stay, a WA CdS must be obtained by 30/06/21, even by those who already have a CdS or who have been living in FR without one, because FR, like GB, have never insisted that a CdS is obligatory. EU FoM rules rule, as far as the FR are concerned.
The procedures for buying a property do not have to be initiated nor completed by 31/12/20. It is not a limiting factor for the purposes of qualifying or not for residency. Clearly, if in process of buying a property come 31/12/20 it might go a very long way to demonstrate intention to stay - but could also be a holiday property.
I saw a puff from a Brit French property portal which featured a Brit estate agent urging people to get their skates on and do the business by 31/12/20. This was simply scare-mongering - this date is not a cut-off point as far as property purchase is concerned.
Yes, one must have established a prescence in FR by 31/12/20. But the FR are being extremely generous in this regard.
I have seen examples on the web pages [as above] whereby a person arrives in FR on [say] 10/12/31 with a contract of employment in their pocket. No problem. You made it in time - you can stay if that’s what you intend.
Another example illustrated that one need not actually be on FR soil on 01/01/21 if one has previously exercised FoM and established intention to stay - by being in FR 90 days+, for example.
Don’t rush your property purchase. The majority of your efforts should be in organising somewhere to get your feet under the FR table.