How harsh would the penalty have to be to make their cheating not worth it?
Taking away all their "ill gotten" trophies and relegating them wouldn't do too much. People would still remember them as the dominant Premier league team that won a treble. And in a few years people would barely remember their short visit to x division. It would just be a stain on their Wikipedia page.
They'll have to disband their current team and start over according to PSR and actual incomes, etc. Send Rodri to ManU and Haaland to Everton where he'll get a real challenge.