For communities, if you haven’t already, it may be helpful to have a look at #software:lemmy’s implementation and the ongoing discussion in standardizing on a common community extension.
With regards to your use of context, I think that would be semantically correct. But I think that what is usually done is to have the topic post being a “root note/page” and the follow ups being replies (inReplyTo). In OStatus there is the conversation URI, which expresses the same that I think you are trying to.