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.
Second, are you hedging your bets and supporting Matrix or XMPP as well, or will you only encourage people to "donate" to the platform that you happen to have picked already?
I also use Matrix. Element has been pretty good for a few years now, but it's still not smooth enough for mainstream use. (Encryption state in chats gets messed up sometimes, for example. It feels like Signal 10 years ago, and it's had security issues in its client also)
The Matrix protocol is also inferior to Signal in that all metadata is stored in cleartext on the server. You get to choose or run a server, but the protocol still leaks the user info to whoever runs the home server and to any foreign server that has a user in the same channel if you are using it in a federated context. Signal manages all of this by peer to peer messages where cleartext is only available to clients, which is really slick.
XMPP is just dead. Forget about XMPP. Matrix is the clear leader in the federated messaging system category. I'd like to see Matrix displace things like Telegram, Discord, and Slack. I may donate to Matrix affiliated projects in the future, as I also donate to other open source projects from time to time, but I'm not going to promote any of those things in this thread.