Put up a yearly donation drive (like Wikipedia) but unlike Wikipedia do:

1. a competition between the various instances, on which collects the most donations
2. not shift the page content when displaying the donation banner!

Ideally the donations will be handled through a non-profit org dedicated to this particular purpose. If the donation level is high enough, developers can be hired to further improve the source code. Currently the funds are managed through OpenCollective, but with enough growth this may not be feasible any longer.

This will most likely lead to heated debates as this will build a somewhat centralized organization, which necessarily comes with power concentration.