Then just ignore its directions when it leads you away from the main road, it can’t know every preference you have for every street. I’d guess generally you’d want it to not use main roads if there are alternatives.
Also maybe have a look if one of the brouter profiles may work better for you, maybe the road bike profiles keep you off the cobble stones, there is a brouter plugin for osmand on f-droid.
That Osmand ain’t good might have to do with the OSM map quality in your area.
I have been using Osmand with brouter as the routing engine, brouter plugin is available on f-droid. You can try out its routing for example on bikerouter.de, top left gives you a bunch of profiles to choose from or even edit them to your liking. Maybe you can find something that works better for you.
Then just ignore its directions when it leads you away from the main road, it can’t know every preference you have for every street. I’d guess generally you’d want it to not use main roads if there are alternatives.
Also maybe have a look if one of the brouter profiles may work better for you, maybe the road bike profiles keep you off the cobble stones, there is a brouter plugin for osmand on f-droid.