In my experience pulling trailers with a front engine bus... the main issue is indeed in the hitch area. The long rear overhang of the bus makes that area hit the ground. I mooshed a couple of parking feet and jacks before I got it right.
So now the ball on Millicent is all the way up where the bumper used to be.
I suggest you start there, and "work your way back". Your biggest issue may be the slope of the trailer floor.
I'm currently using a 25' boat trailer, and the frame is "alarmingly" close to the ground towards the rear. At the tail, just a handful of inches! Yet it seldom scrapes, because the axles are so far back. And then it hits at the low peak of the V-shaped cross member. A V-shaped cross-member in front of the axles has also hit. What I'm trying to point out, is that if this trailer had flat cross members, it might never scrape at all.
So your main issue is the "span" between the bus axle and the trailer axle. Put the ball up high, and you may be all set. A six inch drop at the hitch may very well dig a six inch furrow in some driveway one fine day.
That said, since your trailer is so short, it may wind up running at "take-off angle". Then see if you can reposition the axle to below the springs. But do NOT turn a dropped axle up-side-down -- however tempting it may be for ground clearance purposes. (There is camber built into the axle.) Spacer blocks... taller springs... I don't know. Obviously, center of gravity is always a factor. But a bus is a big vehicle, and I never notice if I have the trailer on or not. You just don't want to be going down the road with the trailer on its side after that last curve -- ten miles back.