I guess this discussion boils down to having a best-practice guide on writing ActivityPub extensions.
If we use linked data, let that be describing LD best-practices. That should likely include how/when to reuse existing props/types from other ontologies.
And maybe rather *not* use those types/props, but defer to creating your own namespaced extension. Because choosing from existing ontologies would mean either standardization on that, or supporting full LD functionality/complexity.