This is the route of the Strasbourg-Brest Diagonale we did in 2014: a bit of that might suit.
https://www.openrunner.com/r/3862045The chunk between Strasbourg & Nancy was the easiest route I could find at the time. The bit south of Nancy is on voies cyclables but from there to around km 70 it's ordinary road, albeit not heavily travelled apart from downtown Sarrebourg, where there's a footbridge to cross. From km 70 into Strasbourg it's voie cyclable all the way. I haven't been on the Waltenheim-Strasbourg bit for years, but the rest is in fairly good repair.
I can recommend La Table du Saulnois in Maizière-lès-Vic for lunch, ditto the Hôtel des Vosges in Lutzelbourg, although there is a hamburger kiosk and an excellent pâtisserie as well.
This is the gallery I made of that ride. The early part was fun but after the first day it was probably the most horrible 1000k I've ever done.
https://pbase.com/johnewing/2014_str_brest