We discussed this problem of larger roundabouts with traversable island it in the German community
see Discussion here (in German) Mini_roundabout
and created this solution:
https://wiki.openstreetmap.org/wiki/Key:central_island:traversable
Background:
In Germany, there is only one type roundabouts in terms of traffic law. These can be large or small, with the small ones having a center island that can be driven over so that long vehicles can pass.
Caution is advised with this tagging when different rules apply in your country in mini roundabouts than in normal roundabouts