Everones financial situations differ. It is this that determines whether you (A) stay on the scheme (have no other option), (B) buy used, (C) buy new. Your financial situation also determies the type (class) of car should you choose to come off the scheme.
I also see people quoting extortine main dealer service costs in their calculations.Depending on the car independent specialists are fully brand trained and in many ways do a better job than a main dealer and also cheaper. There are also respected back street garages that are also far cheaper than a main dealer. Its a different scenario if you buy new, but most offer service packages that reduce the cost. It is also allowable to take a new car to VAT registered garage, as long as they use genuine parts the warranty remains.
Same with tyres there is no need to replace with top end, many mid range tyres go through the same vigorous testing.
I looked at eaving the scheme last december because there wasn’t nothing suitable and high ap’s. In the end I chose to remain, for how long is another question. I also think you need to base your total expenditure over 5yrs, as is now common on the scheme and don’t forget to add ontop of a 5yr lease the ap.