How about having the door jamb removable or hinged .
My son in law made this out of, off cuts for me, works a treat
I would remove most of the jamb, maybe leave 100mm at each side to screw it down, then get some scaffold boards to make a ramp about 4 wide, batterns on the underside to keep them together and bed them on the gravel so the doors clear them, treat them with preservitive and they should last years. You can buy 2nds for around £1 a foot?
slab to the level of the shed floor leaving a 40mm gap so no water runs in the shed, then cut the jamb away apart from about 100mm each side and bolt them down. don't forget you can cross cut a slab to suit most levels.. have a look around where you live they have disabled access everywhere same thing really.
Mmmm...tricky one that mr trimmer...the doors dropping below the shed floor is your biggest problem...how about a "hinged bridge ramp "( just made that up, good init ) screwed to the shed floor, open doors fold down ramp, a ramp up to clear the door jamb and then a smaller one down to the shed floor......or , fix the small ramp inside the shed and just have the bigger one hinged....oh....nice shed ...but whats the hole in the floor for, to let the flood water out ?
The problem I see with trimming the bottom of the door is that it weakens the security of the doors, because then they will just be closing against the jamb, which is merely screwed into place. And if some of the jamb does need to be removed, worse still. Also, there is a securing bolt mounted at the bottom of the door which locks by sitting behind the jamb. Still thinking...