It will be more productive to focus on increasing revenue per user, rather than approaches for penny-pinching.
I hate to be that guy, but I will say it again. The best way to make the Fediverse sustainable is by charging a little bit from everyone that is not willing to run their own instance. Communick would be not only profitable, but a significant contributor to Fediverse projects, with "only" 10k users paying its basic plan of $29/year.