"walk a mile in my shoes" might apply here.
I can't believe how this thread has morphed clear out of the original intent
No matter how great of horse caregivers each and everyone of us are, nobody can tell me they didn't find themselves in some sort of extenuating circumstances they wished to h**l they weren't in.
Anyone that can answer "no" to that, hasn't owned horses long enough or was lucky enough to have a big enough income to quickly erradicate issues. Therefore doesn't have to worry about wading thru muck and whatever else, normal everyday folks often have to suffer thru.
Kind of like the saying "if you have no fear, you haven't gone fast enough".
Believe you-me, there were times when I struggled thru similar issues with my horses and amazingly, it was in the driest of places --- how 'bout the Low Desert of Southern California. It was a long time before things resolved the right and proper way and only then, because a neighbor came and helped me for nothing more than diesel fuel and a case of beer.
Back then, there were no horse forums to get advice/help. I had to figure everything out for myself.
Until the mud/manure issue can get resolved, I'm sure OP will be diligent in administering the needed topicals to get hooves back to normal health. She's a step ahead of where I was in SoCal ---- she has a dry stall
That said, getting back to the footing. It's going to have to be dug out past the top soil and the manure-dirt put somewhere the horses can't walk on it.
Don't fill with sand, trust me trust me trust me, that's asking for more problems the next time it rains cats and dogs.
If you can afford driveway base put that down, followed by standard driveway size gravel. If you want to put sand on top of that, ok but you need something under the sand that will allow the water to drain down thru.
If your area is too big to make stones financially possible, give consideration to some sort of drainage system . Pay attention to the direction the water wants to run-off and make your drains to enhance that natural direction.
If your area is small, believe me, thrush bacteria/fungii will not go away until the top layer of mud/manure has been removed, no matter how dry and dusty you might get. I know this from my own experience
Hope that made some sense