when do you move from a simple task queue to a full blown message queue system

we've been using a simple python rq setup for background jobs, and it's been working fine for our current needs. but, as we scale and our inter-service communication grows, i'm starting to wonder when it makes sense to invest in a more robust message queue system like kafka or rabbitmq. what are the key indicators or pain points that signal it's time to make that jump? are there specific trade-offs we should be considering beyond just horizontal scaling?