"We are thinking we could use Avis/Turo for weekend trips, " .

Just think of the hassle of choosing a car, hoping the rates are not high on the particular weekend you want to take, having to deal with with Customer service representative trying to upsell you all kinds of insurance service , inspecting the car to make sure they don't blame you for any existing dings etc, etc and compare that to just getting in your car and just driving off ...

Also as other's have said, not sure if just it's a good idea to include depreciation cost unless your sole purpose is reach some kind of financial goal post to the detriment of everything else.