Put a gentle "Use Firefox" (or any other non-Chromium-based browser) message on your website. It doesn't have to be in-your-face, just something small.
I've taken my own advice and added it to my own website: https://geeklaunch.io/
(It only appears on Chromium-based browsers.)
We can slowly turn the tide, little by little.
But we / this site only represents a small percentage. 85% market share means there are hundreds of millions, if not billions of users that would have to switch to make any kind of impact.
And you can't do that without being a very large company with an operating system or the most popular search engine or other ways to constantly tell people to use your browser, no matter how good or privacy conscious or whatever your own is.