Even with nearly 300 sponsors our hourly rate is $8 before taxes and from what I know Marijn is doing just fine already. I think it’s in his and our interest to make our work financially sustainable too.
That said, we’re sponsoring other related open source projects, for example Y.js already.
Why not just ask users to also sponsor the ProseMirror project directly? Actually, this is what they are doing on their "Sponsor" page: https://www.tiptap.dev/sponsor/