I bought a folding ramp from Chewy and use a "stramp" at the end on the ground to give it some lift so the angle into the door isn't so steep. The "stramp" is a step/ramp that is about 6 inches at the low end and 12 inches at the high end.
We have 2 English Bulldogs and the smaller only has about 3 inch legs so she's dragging her transmission everywhere she goes.
https://www.chewy.com/pet-gear-one-s...ramp/dp/288501
https://www.chewy.com/pet-gear-tri-f...ramp/dp/212530