Ooof, that Harbour Freight mattock sucks. Been there too, friend. Get the nicer Fiskars one and also a digging bar. Stop when you run into big ass rocks and swap to the bar, it'll lever them out much faster and easier comparatively.

Another option would be to rent a machine. Our Home Depot has little BX series Kubotas for rent, and that would have the job done in a flash. The math worked out in the end for us to just own our tractor, but we do have a slightly larger one. We landed up redoing many of our hand dug trenches.

For fenceposts we use an impact drill with a three foot long masonry bit. We have a lot of t-posts and this is seriously the Best compared to pounding them by hand. Goes right through our sandstone. Have to drag the generator around on a cart to power it, but so worth it.