Good news for webmasters. You now have time till next March to welcome the mobile-first indexing by Google.
Our initial plan was to enable mobile-first indexing for all sites in Search in September 2020. We realize that in these uncertain times, it's not always easy to focus on work as otherwise, so we've decided to extend the timeframe to the end of March 2021. At that time, we're planning on switching our indexing over to mobile-first indexing.
Among many other things, Google wants to use the same Robots meta tags on the mobile version as well as the desktop version.
Google discourages the use of small, low-resolution images for the mobile version. If such images are in use, it might not show up on Google Images or might not be indexed at all.
Are you using lazy loading ? I think Discourse does that. A couple of other mainstream software too. Google says
avoid lazy-loading your primary content based on user interactions (like swiping, clicking, or typing), because Googlebot won't trigger these user interactions.