I think a more ethical way is to have user donations. That way the site is truly community run.

Assuming you're running a site as a non-profit and just want to provide a self-sustaining service (which is usually the case in the Fediverse): something that could be beneficial is to publicly announce the price breakdown of everything that goes into the site: servers, domain name, etc, and also disclose how much money is being raised per month. Maybe even list how much surplus money is set aside for months where there aren't enough donations to fully cover the hosting costs. The goal shouldn't be to guilt trip people obviously, but simply matter of factly stating these figures gives users a sense of exactly where their money is going, which has been shown to build trust and increase donations, and also gives them a sense of how much donations are needed, as most users have an impression that an ad-free site can just keep going without them having to donate, not due to an unwillingness to donate, simply due to not knowing.