I may be a bad one to ask on this because I am persnickety on BMW discount percentages
In my mind, 20% would be a reasonable target, but if it were me I would shoot for ~24%. It really just depends on how much effort you want to put forth as it may take many many more hours of work to get a few percentage points more of a discount. You are also right to consider the mileage, as you take a notable hit on the RV with the higher mileage loaners. About three weeks ago I had a loaner 3 series that I could have signed on at 24%, but decided against it (so these discounts may still be out there). But, with the change in volume in each dealership, you may have to settle for a lower discount percentage to re-define what a “good deal” is.