I run a 2/1 with a touchfree machine (plus detail and used car sales) but i'll answer anyway.
we do our repairs in-house and sub out the things that we aren't capable of doing: welding, electric motor repairs, making hp hoses etc.
the business has several aspects; operations,
marketing, sales, etc.
as the manager, i'd say your talents are best utilized when split between many areas of the business and i agree that you cannot be expected to perform many other tasks well or at all if you are prepping cars yourself all day. when time allows and you are busy and the wash is running fine and needs no repairs then why not prep cars? the problem woud exist when you need to repair or maintain or speak to a customer or owner and those things must take a back seat to prepping cars.
not sure if that helps, but if it were me, and i wanted hours and i was a go-getter, i'd manage the prep help as best i could so they knew that if i had a repair or maintenance or customer service to do, then prep the cars and don't bug me. if the owner doesn't get that, it's up to you to make him/her understand. you want to be indispensable but not because you are always doing every single task yourself. a good manager can delegate effectively (and in doing so keep things humming along even with breakdowns, snafus and slammed busy days) and be indispensable for that reason plus others.