Hey Thea,
thank you very much for your helpful answer and for linking a reference design. 
I tried setting my board up with PTH slots (300 µm annular, 1.8 mm slot width) like you suggested:
I am afraid though that the extensive use of slots to achieve all around plating would lead to a non-producible design. Most of the board outline would be milled away at an early manufacturing stage.
Do I need to interrupt the plated edges more frequently for it to work?
As of now, the design triggers a “critical use of elongated holes” warning and requires contacting support, though the slot width is 1.8 mm (no need for nibbling, in accordance with Plated and Non-Plated Slots).
Best regards,
Yannik