You need at least 2 AC central systems if you don’t want exposed ducts in the great room. I’d think one to serve MBR side and great room, 2nd to just serve the 3 bedroom side, that can be shut down if not needed. You might need to supply great room with air from both ends though.
The problem with mini splits is air to bathrooms , laundry room. I could see a 3 head mini for the left side.
You probably have room in the attic to set horizontal units.
I originally thought I’d set my upstairs unit in the attic, then realized I didn’t have enough room and put it in the corner of our large walk in closet. When I switched to geothermal I now had the garage and mud room, so put upstairs unit in 1st floor mud room, ducts up to 2nd floor attic. (your pond would make a great place to put loops if it was closer to house).
Geo units are great for DIY, as the units are closed systems and all you hook up is water loops and ductwork. ( I bought my units from CMC supply In Roanoke VA and highly recommend them) (they delivered to Harrisonburg)