There’s a reputable independent VAG specialist near me who I used once to service my Seat.
There was virtually no difference in cost between them and the Seat main dealer.
On a service plan I’m fairly sure there will be little cost difference.
I had intended, by this point, to change to an independent- I saved a lot of money and got better service when I changed to independents for the BMWs I owned previously- but frankly, I don’t feel the need to change - for now at least