Good morning. I know that area pretty well, although never have visited Gunzburg so can't speak about there.
My two suggestions (and a bonus third) are not in the middle of this short drive but rather the first and bonus are towards the top and the second is towards the bottom - and I feel, for me anyway, all are worth hopping off the autobahn for.
Before I start, the A7 is undergoing some rebuilding both north and south of Rothenburg so be prepared for narrow lanes and subtle signage for Rothenburg. Maybe by the time you get there they will have moved on but when I was there at the end of last summer the junction at Rothenburg was being completely rebuilt and traffic was on temporary roadways - it didn't look like it would be finished anytime soon.
My first suggestion is just off the autobahn after it has crossed the A6, and that is Dinkelsbühl. A walled town with a great town centre, and they have storks living on the roof of the town hall, with video feed down to the street. I think of it as Rothenburg without the crowds. https://en.wikipedia.org/wiki/Dinkelsb%C3%BChl A Disney film was shot there in 1962.
Continuing down the B25 is the bonus - Nördlingen is another walled town, with the distinction of having been built in the crater of a meteor. Very different. But because you expressed a need to keep moving and Nördlingen is a bit further away from the autobahn I have called it a bonus.
My other suggestion is Ottobeuren which is 11 km southeast of Memmingen. It has the amazing baroque Ottobeuren Abbey, and the interior is breathtaking. https://www.abtei-ottobeuren.de/ Full disclosure, I have a connection there, but I would recommend it even if I didn't.
I hope you enjoy your trip.