for yard thats full of mud to drive wheelchair accross ?
I made a pathway from scrap pallet boards. I cut them the same width, then I took an old garden hose and stapled it down a few inches from both ends of board in several places. I placed a scrap in between the boards and stapled again. When it was the length that I wanted, I turned it over-pathway complete. Perfect when we have a lot of rain and it sorta roll/folds up when I don’t need it.
Fill holes with gravel/rocks. Overlay all with gravel or pour cement to cover area at least 2 inches wider than width of chair. Cement needs to be at least 2 inches deep to withstand constant use, so you'll need to build up sides with 2x4. Other option is to build ramp above the mess Some places have pre built sections of ramp(lowes, home depot, etc)
How about small pea gavel
How about a boardwalk? This what they have in the arctic where mud is deep after the melt..... Here is one.... you can use small logs for the base, and even pallet wood for the top. I would add a raised edging on the sides to prevent chair from accidently sliding off the side. https://www.youtube.com/watch?v=ysrdl8GcmQU
For something as serious being used with a wheelchair, I suggest a cement walkway. You can get several bids at Home Advisor.com. Have your measurements ready when you ask for bids.