I'm curious what the breakdown of donations is. I only have 1 contact with a $10/month and 1 with a $5/month badge. Of course there could be others not displaying the badge. Signal really needs 500,000 people giving $20/month and plus the rich guys giving some millions on top of that to be in a safe financial position.
Maybe something that could be done to encourage donations is have the client estimate how much raw infra costs your usage created and display in the donation screen.
Given that Signal is free as a service, supporting federation only increases their expenses.