I'm thinking about how to get rid of those errors, this might the solution but I think there isn't a way to check the list of defederated instances, one thing I'll implement soon is to expire synchronization tries after x days so the list won't grow forever
Great idea but for now I'll just log to the user what couldn't be synced, implementing something like this would be hard for now, maybe a future improvement
It's made in flutter so I can probably port it to Linux and web, once I do it I'll update the app