It would take a fair amount of work on your part. There is no automagic way to migrate a site that complicated, and no obvious path of “you’ll definitely want to use XYZ SSG” - you’ll have to evaluate what works well, figure out what can map to your current setup, probably resign yourself to doing some redesign work to accommodate the best-but-not-perfect featureset for your use case.
This is a demo of how it CAN work, but again, without custom plugins and private user accounts: