2M users but no money in the bank. Tough times

I think you don't get to want some amount of advertisement. If the project requires it the pages should be full of them. You have already explained that money is running out. People can donate or view ads.

If things are not niche enough perhaps split off some part that can be niche enough and improve it.

Looks like this is a completely free project, 100% supported by donations, and no option for a paid plan https://exercism.org/donate

This is one case where I wonder if the founders could keep the 'core' free and open source, and have a paid model on top of it to generate enough revenue to work full-time on it.

Apparently the donations cover the server costs.

So you have 2 options. Keep it free and open source and have the community develop it.

Keep wanting to get paid and have employees, meaning you need to either start charging money (perhaps for “pro”) features, show ads (please don’t), or do a better job with getting donations.