Relegation is nothing.
They will be instantly promoted and all it does is punish the Championship teams because it's effective a write off for one peomotion place.
Strip them of their titles. That's the only thing that punishes because that's what it was about, that's what they gained.
Any financial penalty will be simply the cost of doing business because money doesn't matter to these. Relgation is a one season bump. Point deduction is a one season bump and even more a gentle slap than relegation.
Remove their titles and black mark every record they registered
Unless Man City are stripped of titles then any punishment is not tough
A fine is just business and they will just invent a new sponsor
Relegation is a year out while they stomp the 2nd tier. It also punishes those teams because there will be one less promotion place available because Man City will obviously go straight back up
A point deduction does even less than a relegation.
Take away the awards they "earned" through cheating because that's what it was about. On field success. Unless that is taken they have effectively gotten away with it with a mild to soft slap.