A deck mower will level it all but you’ll still have a pile of slash, but more skid loaders can run one. Probably the best option is a forestry mulcher on a high flow skid steer loader if you do it with a skid loader. You can get it all into the topsoil layer, unlike a deck mower or a disc mulcher.
Disc mowers are technically a mulcher but they’re a lot smaller and don’t do as good a job as a proper drum type FAE/Fecon style. The reason for this is the small disc can be run, again, by a smaller machine with less horsepower.
I liked an FAE head on a Cat 299 XHP when I was doing that type of work, and wanted to limit the footprint I left while still getting a decent production number. If I really wanted to get in there a 20-25 ton excavator with a big flail mower would knock that out in a hurry, but also make a mess underfoot. The least intrusive way to do that would be a 3-5 ton excavator with a flail mower, they have good reach and can reach into tight spots.
For mulching, Cat’s XHP package paired with it’s forestry guarding is hard to beat, Kubota’s big CTL is very good and ASV’s big machines have always been built for forestry mulching. Whatever you get, just make sure it has a lexan door, and maybe some extra guarding for your rad and lower track rollers so you don’t jump a track off.