Wow, you’ve got your hands full, you seem to have a good handle on the priorities so I’ll keep it brief.
For septic I’d go with what’s called an infiltrator system, it’s simpler to install, less digging and it eliminates the multiple runs of pipe common in older systems.
Your forestry mulcher was a good idea, I’d broadcast something like orchard grass to cover the area until you can fence it off and/or put it into food production, you’ll be amazed how fast Mother Nature will want to reclaim it!
