As
@cougie uk says , use your Wahoo for navigation, it's what it's for. I'm not a Wahoo user but I imagine uploading routes should be easy enough.
As for route planning apps there are quite a few, and all pretty good, so whatever you choose you probably won't be disappointed. They're all much of a muchness. The main thing is easily syncing routes with your Wahoo, and I can't help on that.
There's Komoot. Strava has a route planner but I think it's paid subscribers only. There's RideWithGPS which is my personal favourite. cycle.travel often gets mentioned on here and the author is a CC member, and I've played around with it and it seems OK. The only one I actually don't recommend is OSMand which I found positively user-hostile when I played with it, but that may be because I'm a bit stupid.
The main thing to keep in mind is you have to engage your brain when doing route planning. Plan the route in your head and let the app follow you, rather than the other way round. If you let an algorithm do your routing for you then from time to time it will do something daft or miss an interesting opportunity to see something, so examine your route carefully on the map and understand where you're going.