Your first mistake is paying taxes in the UK when you don’t live there. UK is a territorial tax country. It sounds like you need to hire an accountant in both countries. Don’t try to figure this stuff out for yourself, you will just end up paying more. I have some expérience dealing with HMRC snd they are very difficult to deal with.

Yeah my understanding is that digital nomad visas let you stay in the country that issued it only. For the rest of Schengen you get 90 days. If anyone could travel with a temporary visa there would be no reason to seek citizenship.

As far as which one, I would avoid Portugal since it seems like Portugal is getting very crowded with expats. I have heard that Greece is one of the easiest ones to get.