The truth is there is very little by way of 'interesting' (whatever that means) roads * between Calais and Stockholm, unless you work by way of making them interesting. At some point you inevitably have to cross the Rhine, which will bring you into the flat lands of the north German plain.
Suggestion:
Calais to Stockholm is roughly a straight line, north east, along the axis: Calais, Hamburg / Kiel, Copenhagen / Helsingborg, Vaxjo, Norrkoping, Stockholm.
You'll be passing by (or close to) Arnhem, so you can take a day to look at the WW2 museums, if that is your thing. Or skip that and motor straight to Lubeck, on the north German Baltic coast.
Lubeck and its other sister Hanseatic League towns, along with the Flensburg German peninsula and / or along the Baltic coast eastwards as far as Stralsund and Rugen island are all 'interesting' enough and not much traveled by Brits.
There again, few Brits go to Mongolia. Some kind soul (that'll be me) put up some suggested routes around the peninsula / along the Baltic coast:
https://www.ukgser.com/forums/showthread.php/494659-German-Baltic-coast.
It really annoys Tim O when I do it
as nobody must ever be told or suggested a route but hey-ho, that's life. Or you can go and look at one of the many other (small) islands that Germany has along its coast. It's up to you. Obviously quicker to get around using the motorways or main roads or you can bimble about, dreaming of the Mongolian steppes.
At some point you do still have to get from Germany to Denmark, which you can do by road by going up the Flensburg peninsula to Kolding, then turning east via Odense to Copenhagen'ish . From there you cross to Sweden, either via Malmo or Helsingborg. Alternatively, you might find there are ferries that cross directly from Germany to Sweden, via Kiel, Lubeck, Rostock or even perhaps, Rugen; you'll need to check. If there are, you can miss out Denmark. Either way, you are now faced with the near enough flat run to Stockholm. About the only thing of 'interest' (other than Sweden itself) will be the plethora of lakes around Vaxjo.
Me? I'd get from Calais to Hamburg / the German Baltic coast as reasonably quickly as possible, maybe having spent half a day in Arnhem on the way, if time permits. As you don't tell us what time you trundle into Calais, you'll have to work out how best to do that. Then, spend a day on the Baltic coast or working your way up the peninsula.... or working out how to get a ferry to Sweden... then spend the three days'ish that are left getting to Stockholm.
*Actually that's not true, of course. You can find 'interesting' roads anywhere but you have to work at it. North Germany is 'different' to the Alps, as it's flat. That being said, the cross-country roads can be 'intersting' as they are different to say, the Ardennes.
Stockholm is about 2000 kms away so do it over a 4 day period 500 kms a day nice and easy, will give you a 2 day safety net, and predict tyre change maybe somewhere, now will let Mr knowall Wapping snipe in with his clever crap.
