You are correct - there's no programmatic transfer of workflows between FFProM and FFCore (the underlying infrastructure is completely different in FFCore).
Yes, you would have to build the workflows from scratch. One thing to keep in mind - the FFCore versions of the workflows should be simpler (in some cases dramatically simpler vs the FFProM versions of the workflows). Workflow simplification has been a big focus in FFCore so I would really understand what a workflow does and then look for the simplest way to implement the same thing in FFCore. In my opinion, the simpler your workflows the better.
There's no way to extract documentation from the existing workflows. I would suggest opening the FFCore UI from the FFProM server and building workflows one at a time by looking at the current FFProM workflows.
If I had to give any piece of advise it would be to note that there's no huge rush. I would focus FFCore on any new jobs that are coming and move existing workflows at a more measured pace.
I see a post from almost two years ago saying there is no programatic solution to migrating from Process manager to FreeFlow Core. What does the manual process look like? Do I need to rebuild the workflows from scratch? Is there a way to document the workflows from Process Manager to make this process easier? The person who built the Process Manager flows is no longer with the company, and he left no documentation behind. Sorry for so many questions. I just want to get started on the right foot.