OK, I will try and help out the best I can. I can't tell you much about fees as we have direct billing at most places.
Erfurt is quite straightforward. I don't think you need handling there. They have a dedicated G/A facility so you can avoid the terminal. Should be good for a quick turnaround.
Haven't been to Altenburg but should be straight forward aswell.
Whenever I was in Luxembourg it has always been a bit of a mess because you needed Luxair handling to drive you to the terminal and back etc. The landing fees are moderate but I think handling fees probably aren't. however, I seem to remember that they were in the process o opening a dedicated G/A facility so things might have changed. You might want to give them a call.
The airports in Belgium are not bad, but again, not sure if you need handling. We always use a handling agent but I woud try and find a place where it is not needed for a fuel stop.
I recommend you phone up the places and ask about fees and the need for a handling agent. ATC and infrastrucure (RWY, Nav-aids etc) wise all the places you mentioned should be no problem.