> The problem I have most of all with the routing is that it seems to default to my location as either the From or To value (whichever is the one I haven't selected). Once that's been chosen it's not immediately clear how to change it

Yeah this is an oft-asked question at least for OsmAnd, but I was recently corrected on this myself: so long as you don't press 'start' on the route, you can always change the from position freely and it won't flip back to your current location. You can close the navigation window (back button, swipe it away, or tap on the map) to get the map view back in full-screen for further browsing and selecting another point that you want as departure point, waypoint, or (subsequent) destination. Rinse and repeat until you have the route figured out with all the waypoints :)

I use this a lot for planning mapping routes. Another protip is to go into the waypoints view and on the top right you can do traveling salesman sorting to get the shortest way that visits all waypoints

CoMaps/OrganicMaps I don't know; I'm one of those who grew up with OsmAnd and got used to having all those features