Your images are kind of small and fuzzy, so I'm just responding to what I'm imagining I might be seeing
Looks like a clever solution!
Now, remember, each shroud will be tensioned to, say 15% of its breaking strength, or around 300# So, your mast-jack has to be able to handle 900# of compression, or some higher value (to include a safety factor). On my DS, the mast jack is a 1" threaded rod, with a bronze nut that itself is about 1" thick.
I have to keep the rod and nut well lubricated, or else the friction is difficult to overcome. The nut for the mast base is circular, with about 3" diameter, so that it pretty much supports the entire mast foot. The mast foot has a casting with a circular opening in it, that centers the mast on the jack, and it provides a good transition for the load from the thin aluminum walls of the mast profile. However, by extending the mast jack out as far as it goes, the mast foot does not have to distribute the load from its center to its rim, it mainly supplies a flat surface. There's a washer or two to make sure that the jack doesn't scrape at the mast foot while being turned.
The mast jack does not use a wrench, but has holes drilled into it for a short handle that is inserted. Mine looks a bit "aftermarket" as it resembles nothing so much a nail punch (used blunt end first). Looks like one of the POs misplaced the original
Other than some worries about how sturdy your setup is, the basic configuration you have in your image looks about right. Needs some heavy washers. I also don't know how well the galvanized would work when loaded. I'm thinking friction here.