On the Fireside Fedi interview with Jerry ( the admin of Infosec.Exchange Mastodon instance ) a scary truth was suddenly revealed ( on 34:11 ): Just to keep the instance up and running he needs to spend up to $5000 a month, pretty much out of his pocket. Donations to the instance barely cover any of that. And if he will ask people to pay to use it, they will, rightfully so, switch to a different instance.
Freemium is the way to go. All the essential features are free; you can pay for extra stuff like special emojis, coins(like Reddit silver/gold), or customizable profiles. It could be either a subscription or à la carte.
Simply giving something in return would incentivize people to donate more.