I recently changed the oil on my car. I chocked the rear wheels and jacked up one sill at a time using a trolley jack with a rubber pad. I had two axle stands and put one on each side under the subframe. There were parts of the subframe visible and usable (at the back) while the undertray was still on.
Give the car a good shake from both sides and check it feels secure before putting any part of you under it.
I was barely under the car at all. I found that most of the undertray fasteners were reachable without going fully under there but I guess it maybe depends how comfortable one is rolling around on the floor and how long one's arms are
. I'll also confess I have several undertray fasteners missing so less hassle.