You are right, there’s no documentation yet, and the implementation still has many missing parts. It will take a few more months until it’s in a state where I’d recommend using it for an actual migration.
The first converter we’ll be shipping initially will be a “Discourse” converter. It’s intended as a reference implementation and replacement for the current Discourse merger script. We’ll might ship an SMF2 converter in the future, but it’s not at the top of our list. So, you’d either need to create it yourself, wait for a contribution from the community, or hope that a customer pays us to migrate from SMF2.
So, if you want to do the migration right now, I suggest sticking with the existing import script and maybe adding the missing features you need for the migration.