50hp like you have seen would blow your 15k away and ask for 15k more lol.
My buddy has a 28Max with a backhoe and FEL, it's a 2013 model I believe, it's a very stout machine but only weighs around 1800lbs I think.
The problem that you face is, your expectations are really high for a little machine, my buddy with that 28 max did the same, within the first week he talked about trading it off for a bigger 1538 tractor because the 28 max couldn't lift chunks of concrete sidewalk for him.
So far he still has it but my point is that these little tractors are little lol they will get the job done VERY slowly.
I have a 4320 which is 48hp at over 5k lbs and I wouldn't think it would be very useful for heavy ground work, I feel even at its weight would spin tires with the R4 tires.
To get a smaller tractor to pull harder Tires, Weight and 4x4 is very important, R1 tires are gonna pull hardest and turf prob the worst.
Grading existing drives, a land plane is very effective, faster and more efficient than a box blade but the box blade digs better on new ground.
If I had to dig deep in soil I would buy a 2-3 bottom plow and disk, fastest way to dig up soil with a tractor I feel, plow to dig, remove dirt with FEL and smooth with a 6-8ft disk.
I have never graded with a tractor but that's how I would approach it.