Mike, I agree, there are some good ideas in there. Those rides though are street rides mostly aimed at the Harley guys, and most aren't very close to Jacksonville. The ride closest to Jacksonville in that link, uses I-95 most of the way. Please keep looking and come up with other ideas close to Jacksonville, that includes some legal trail riding as well. Something on back roads that leads to a State Forest or State Park that allows street registered motorcycles on firm trails there, or else something within a State area that allows registered vehicles on the roads. I think everyone shares your view that they want to avoid mud and water, and I would also add to try to avoid deep sand.
Gainesville is pretty close to Jacksonville. Maybe find something about halfway between Gainesville and Jacksonville, so that none of us including yourself need to trailer to the meeting point for the ride.
I have never been to Jennings State Forest, but here is a map that has some promise?
https://ccmedia.fdacs.gov/content/download/27859/file/Jennings-State-Forest-Entire-Forest.pdf
While much of the trails there are hiking and horse trails, I think all the trails in black are good roads that allow registered vehicles. You can also use Google Street view on some of the black roads, and they look to be ok. 217 is paved, Long Branch is some paved and some dirt, hard to tell about the sand there though.
Also look at Belmore Wildlife Management Area, and also all the other green spots on the map between Gainesville and Jacksonville.
https://ocean.floridamarine.org/HGMSearch/BrochureDetails.aspx?srctype=pfs&title=belmore
You have to research the access fees too. Most of the wildlife management area's fees are inexpensive but some are not.
Glad you have volunteered to lead the event. Much appreciated... count me in no matter where it is.