Folding step?
Is the answer basically.
For the few times I’ve used a roof rack setup on our C4 pic, yep standing on the door sills is not a sensible way to lift and or work on bolts and straps.
‘Technique’ wise hold the bike lower down the fork leg and seat tube then you would normally, whatever your stepping on you need to be able to get a foot onto without obstructions or anything. You’re going to lift the bike, and in a single step forwards go up. The main goal is to get the wheels into the carrier, one they’re supported you can pivot the bike up and mess about with clamping arms etc (depending on the carrier)…
If I was transporting multiple of bikes frequently (say weekly?) I would certainly want a tow bar mounted rack; I’d not be fitting/removing the roof bars that often, and I’d not be leaving a roof carrier on the car permanently. Doing multiple unergonomic overhead lifts of 30lbs+ in various carparks, you’re bound to injure yourself eventually… YMMV.