Perhaps a #monolithic tag would be appropriate for software which does not function as a pure AP Client or Server, but instead simply communicates over S2S while subsuming the C2S part with internal business logic.
Tags are indicative of compliance: if your implementation complies to C2S, or aims at such compliance, use #client; if it’s S2S, go #server: a single category may be listed in one or more tags. “#monolithic” doesn’t really convey anything trustworthy. 
In the example you gave, I would use #server only (since the C2S part is “internal”). The idea is that developers can compare implementations: so if you’re looking for C2S implementation, you would head to the #client tag. Maybe we could rename these tags: #c2s, #s2s but this maybe become confusing with #activitypub:c2s and #activitypub:s2s.