Mass transfer apps to adopted storage
Okay, after some semi-random googling I found the app that will do the job. Apps2SD. It's free, requires root but does batch move apps to adopted storage. It's a bit slow too but beats having to do the "transfer to external" sequence for each app hands down. 

For the interested, googling for app for adoptive storage doesn't give you anything useful but googling for link2sd marshmallow links to a XDA thread that mentions Apps2SD as a workaround for Marshmallow. It can create mounting scripts unlike Link2SD but Link2SD will then happily use the 2nd partition if you like it better.