Most things are no issue for SSB. If you don’t intend to use email, you can turn off email functionality for the domain:
That will spare you the warnings in diagnosis.
Hairpinning might come into play. Hairpinning is when the router recognizes you want to reach your (web)server from inside the LAN, and lets you visit it directly on the internal IP instead of going to the internet first, and then returning to your home LAN. When you ping your Yunohost by its hostname versus pinging it on the internal IP, is there a large difference in reply time?
Debian got a new version, ‘recently’. Since the image was created, the apt configuration is outdated. Are you familiar with SSH? Then have a look at this post on how to correct the configuration.