I've spent the last week mucking out my friends barn, 14 inch thick (so roughly the same as OP) layer of compacted hay and muck. It was intense, but also kind of satisfying. Hay fork was my favourite tool, just stand on top of the hay and pry up the layers, like pulling up a carpet.
I read, I think it was polyfacefarm that some people sprinkle dry corn kernels before adding each layer of bedding, then in the spring, turn the
chickens and/or pigs into the barn. The chickens/pigs/whichever it was, dig up the manure looking for the corn. I've never tried it and I have doubts as to how well it would work.
Other people use different bedding material, or add it more often, but again, it seems to get compacted down.
If I had my druthers, I would design the barn stall to be the same width as the front end loader, with the door at each end the same width, so I could just drive the
tractor through and scrape out all the muck - but life seldom gives us the chance to have our druthers.
Or you can do like my friend did and offer free manure on UsedAnywhere - you muck out, must take all. If your fellow farmer friends see you offering free muck, they will come around in droves and steal it for themselves.
That's my brainstorming done for this morning. Can't wait to read what tips and ideas other people have.
Edit: Did I mention that my manure pile is now 6 foot wide, 7 foot tall, 25 foot long, and I still have at least two more pickup truck full to bring home. That's all from one barn and does not include contributions from my own farm animals.