Depends on what you mean by advertising really. We e-mailed local draughtsclubs about the site's existence for example, and lidraughts was placed as a link on https://fmjd.org/?p=online because we asked. There's nothing wrong with that, but it's something else to spam random people or to literally advertise.
We plan to get a patron / donation system in the future, it would certainly help covering server expenses
The solution is quite simple. WORK TOGETHER. There are plenty of sites you can play draughts, start working together. Think of playok, DamZ.nl etc. This site is also an example of how things should not be done. The site on itself is great. A lot of people have great ideas about how draughts can become bigger. Don't invent something new, but first work together and improve what is already there.