The broad brush questions are the hardest. BMW is probably the best to look at over time because it’s one MF across all models
Nothing should follow the Fed rate since that’s overnight to banks, but you could draw a line with 2/10 year Treasuries and roughly see a fit.
If you look at Ford, some vehicles have/had very low interest rates and the trucks are still 6-7% on Tier 1 because the people who are going to lease a Ford truck will get pay it: no need to buy it down.
As long as there is more demand than supply, no reason to make MF any more attractive than whatever the cost of capital + standard markup is for the financial arms, they are a business too.