I do hope other more knowledgable members than me will along to advise further!
My comments are largely based on working with the HCx - the major relevant limitation being the limit on the number of "places" (I'm using that word to avoid getting trapped in the minefield of different sorts of "places" that tracks and routes can involve) that it can handle in a route. By creating an "off-road" route, i.e. as the crow flies between the places you have specified, you are making it easier to stay within that limit, whereas with "follow-road" by necessity that limit is reached much more quickly, hence your truncation.
Anyway, there are loads of ways of doing this, but the aim is to have a set of tracks on your device, each corresponding identifiably with a manageable section of your tour - per day should work which follow the roads of your intended journey. You can select the appropriate one for that day and ensure that it is displayed. The tracks are to be used as a passive guide to the actual roads you planned to follow. No matter how f-up your route might get, your track is intended to remain unchanged as a reference {I believe the E30 is cleverer than an HCx and may be able to direct you back to a track, but let's not go there for now).
For each of those tracks, you create a route. The route is an active assistant that will direct you in the direction of the next "place" you have created. Ensure the route is within the limits of your device. On an HCx, that limit is so small that follow-roads will not work for anything other than very short trips, so it is best to make them minimalist - "places" at all essential points, junctions, bits that look potentially confusing (create a "place" on the correct fork in the road, e.g.). The E30 may have a higher limit so you could afford to be more generous with the number of places you create and name.
The other thing with creeating your own route as above is that it forces you to look closely at the map. When you zoom in to urban junctions, let's say, you can see the potential for error and can bring in satellite view to clarify, add a usefully located / named "place" and generally it's a good way to really get a feel for what you are going to see in front of you.
Whatever you do, unless an E30 is a drastic improvement on an HCx, don't rely on the device to create your route from scratch or destination-only. There is a high risk that it won't take you the way you want to go. You need to force your route to go where you want it to go by use of carefully chosen places that you create and name yourself.
