I think this is still to be properly established. The current practice is not to stamp passports of UK citizens with such retained rights. It remains to see how this develops. As you say there is little to stop you crossing the internal borders without being checked. Even car number-plate scanners do not record who is in the car.
Even if they do clamp down on this then unlimited time in your favorite country plus 90 days in 180 for the rest will be far better than only 90 in 180 for everywhere. If that happened then we would do long shuttles back and forth from the UK to Greece - resetting the clock at both ends.
And if you can't get residency in any EU country then I believe more will offer easy-to-get long visit visas. Here is one I just spotted for Italy....
Short Stay visas In accordance with the provisions of the EU Regulation 2019/592, starting from 1 January 2021 (the end of the transition period) the United Kingdom will be added to Annex II of the EU regulation 2018/1806. British citizens will therefore not need a Schengen short-stay visa to...
conslondra.esteri.it