Steve’s Yardmax looks like the solution for his situation, with the hill and load. We’ve tried a couple of other ideas that probably won’t do hills like his, but have been useful on our flatter terrain. One is based on the old Chinese wheelbarrows, with one wheel centered in the cargo area. Here’s more pictures than you need, but interesting:
https://solar.lowtechmagazine.com/2011/12/how-to-downsize-a-transport-network-the-chinese-wheelbarrow/
My son built something similar using a pallet with a 26" bike wheel centered under it. He cut the rear triangle off the bike, screwed it to the pallet, and added some bracing. He also added handles, and rear feet so it stands by itself on reasonably level ground. I’ve had 150 lbs, say 70 kilos of feed on it. Only once 
. But for 50 kilos of scythed hay, it works really well. Unlike the Chinese barrows, all the weight is above the wheel, so it does take some balancing. But it will go any place your feet will, as long as there’s room where the pallet is.
I wondered about two wheels to avoid the balance issue. I put wheelchair wheels on another pallet, with axles centered on the edge of the pallet, and added a U-shaped handle like a garden cart. No worries with balance, but less nimble, and more work since you’re pushing down instead of forward. It’s a little easier to load hay, since it’s lower. Weight capacity is similar, with this one limited by the wheels’ attachment strength instead of limited balance.
My son can handle more weight on the single-wheel barrow, since he’s a little taller and a little (at least a little) stronger. If the handles were set lower, balancing might be easier for me.
One last crazy idea. I found this looking for wheelbarrow ideas and photos. I won’t spoil the surprise, and I’m not sure what to do with it. Maybe I could haul firewood up hill, out of the narrow greenhouse door. FWIW.
https://www.youtube.com/watch?v=PTJOxyz6nd0&t=381s
Have a blessed Christmas if I don’t bother you with any more crazy stuff!
. But for 50 kilos of scythed hay, it works really well. Unlike the Chinese barrows, all the weight is above the wheel, so it does take some balancing. But it will go any place your feet will, as long as there’s room where the pallet is.
