"it allows people with PR in one member state to move to other participating member states, despite not being citizens. It's a huge paperwork issue apparently but it does exist"
I'm sure that is NOT the case.
Please supply any link you have that states otherwise - from the EU, not a hopeful article from elsewhere
Most, if not all, of the E27 have said that UK expats can continue to work in their E27 host country, including providing some services
BUT
there would be no FOM - even under the WA -
and I'm pretty sure, no right to sell services if there is No Deal
That's the problem with No Deal - the UK would be treated as a 3rd country as far as the EU is concerned, including our expats.
UK cross-border workers are one of the groups that are known will be severely affected after No Deal.
They will be dependent on any agreement between their host & neighbouring countries - which could take some time
It is also unclear after No Deal under what circumstances professional qualifications for UK lawyers, doctors etc would be recognised, e.g. those obtained before Brexit Day
- that probably depends on the host country and probably won't be EU wide
Under the WA, UK expat professional qualifications would be recognised throughout the EU.