I use osmand for privacy but I think it just emphasises main roads. In Melbourne it always suggests turning off cemetery road west because it doesn't know it's congested and will get me stuck for 20 minutes. And there are some missing slip roads. And navigation constantly fails to start. I wonder, how difficult is it to make minor edits to the map data?

> I wonder, how difficult is it to make minor edits to the map data?

The map data is OpenStreetMap, so you can make edits via the standard OSM methods:

Web: https://ideditor.com/

Local: https://josm.openstreetmap.de/

It's just OpenStreetMap, see https://wiki.openstreetmap.org/wiki/How_to_contribute