Whatever tool you use, the most important is your brain.
I routinely plan rides of hundreds of km in unfamiliar areas using mainly Komoot. It's very reluctant to route along primary roads, quite reluctant to route along secondary roads. That's the UK terminology but also the classification used in OpenStreetMap, the data source for most route planners.
I use a lot of manual control points, dragging the route.
Touring mode can be interesting, but it sends you down unpaved stuff which can be really rough, without highlighting it. Road cycling mode warns you about unpaved stretches. Flicking between the two can be instructive.
Pressing M on your keyboard hides the route, and lets you see the underlying road colour code. White is used for the nost minor roads on the OpenCycleMap layer, which is much clearer than the default, especially if you're looking for (on-road) cycle routes.
Long straight roads and those passing urban areas are worth checking, perhaps even using Google Street View, but that can miss busy periods.
I tend to have Google maps and OpenStreetMap open at the same time as Komoot for planning major trips.
For a big tour I'd break it into logical sections of a couple of hundred km or less, so a section can be planned snd put to one side.