Using a grinder with a 4" diamond blade, could you cut a
v shaped groove or notch the width of the door to drain
water to lower ground?

I'm afraid if you adhere anything under the door, the
stress of driving over it over and over will break it loose.

Almost 20 years ago I tiled a garage floor using brick
pavers (1/2" thick brick), thinsetted with epoxy thinset.
It is still holding up as I see it when I work in the old lady's
neighborhood. A few months ago the door was open so
I asked if I could see it, and sure enough it's still bonded.
Says a lot for epoxy thinset.

thetiler