Forget the Black Forest if you’re going to Colditz. It’s in the wrong direction. Either go and enjoy the BF or Colditz.not both.
There’s enough nice roads in the BF to fill a few days.
Yes, the obvious advice would of been "that's in the wrong direction" and it took the Tosser experts 15 posts to notice, so qhat quality of advice is the OP hoping to get here anyway?
It's like saying I want advice on a nice route from Birmingham to Newcastle using the best roads in Cornwall.
As for Black Forest, most of the roads are nice enough, any mapping that shows the built up bits will give you an idea where to skirt around if you can, Michelin maps will show the "scenic" green routes, which tend to be nice and curvy, I prefer My Route App for planning as I can use Google, Michelin, Garmin, Tom Tom map overlays which all have pros and cons so I often switch between them.
B500 has become a bit shit, 20 years ago it was pretty good, mainly 100kph and not as much enforcement, 10 years ago it had a lot more 70kph sections appearing, a couple of years ago and it was nearly all 70 or 50kph (44 / 30) along most of it (well, the famous bit which is the 25 miles south of baden - it runs right down to Swiss border)
If you do detour via Black Forest then go see for yourself, try and avoid weekends when it will be busier and have more plod attention, early morning mid-week is probably best best to run along at an enjoyeable pace without getting locked up.
From Luxembourg you could also head directly east and ride through the Hartz mountains, a small area I have yet to explore, but looks good and won't have the same level of ultra low speed limits and plod focus (I assume)
Ardennes and Luxembourg are all good roads, from there you can ride across Eifel (I have less experience here, but roads seem quite good, but (IMHO) not as good as Luxembourg) and then from there onto Hartz - beyond that point I have zero knowledge, but mate went out that way a few years back and said still some OK back roads - and worth booking a Colditz tour guide.
Personally I would save the Black Forest for another trip, easy to make a good Black Forest trip in under a week, and Vosges are next to it, with Luxembourg to the North, so you can go out to Vosges, have a day riding around there, cross to Black Forest and have another full day looping about there and then return home via Lxembourg and Ardeness (or vice-versa) making for a really good 6-7 day trip, or use a weeks work holiday and mke it 9 days with 3 nights in Voges and Black Forest and night on way out / back in France / Luxembourg.