So generally for backends you make your code available under the agpl version 3. And so anyone who is running your relay software must provide the source code and installation instructions to anyone who has accessed it.

That gives users a great or feeling of confidence that there is no malicious code.

If you really don't want others making money from your work you can provide the source code under a Business Source License. Which allows users to few your code but not run it for business purposes.

I understand you feel your work is being stolen by sharing it but tbh, you yourself can't really compete with a bunch of FOSS devs. I make pull request to FOSS projects all the time. So open source relays are gonna grow to be better than your code quickly anyways.

The best way to make money is to provide a superior service imo.

Look at grapheneOS. They have 6 full time devs all from donations. And want to hire 6 more