You are going to struggle to find a flattish route out of Rotherham past Sheffield and Chesterfield. Also this is outside my area, but I can advise beyond that.
If you can aim for Donnington Motor Racing circuit, then to Breedon-on-the-Hill (OK, a hill there, but from memory, the town is mostly at the bottom), then Nottingham Road via Lount (sorry - a bit of a hill here) to Ashby de-la-Zouch. Out of Ashby towards A42, and go under, then head towards and through Measham, No Man's Heath, Seckington, Tamworth. At Tamworth, either get on the Bimingham and Fazeley Canal (a bit rough surface) or chance your arm on the A4091 past The Belfry. Next up is the massive J9 on M42/M6 Toll start. Either find the canal quick, or take a deep breath and fight all the traffice including heavies to get on the Kingsbury Road to Minworth, heading for the Midpoint Park distribution centre. Yep, lots of big lorries around here focusing on getting to the depot. I have only hit this around 21:00 in the evening, at which point it was just about reasonable. Any earlier, and you want to be on the canal, which fortunately takes you to Spaghetti junction. As does the road route down Kingsbury Road and Tyburn Road.
From Spaghetti, either the Lichfied Road or the canal (Grand Union or Birmingham and Fazeley - choice is yours, but I favour the latter) will get you to central Birmingham. Come off at Fazeley Street in the heart of Digbeth to find your way to NCN5 and the Rea Valley route. This will take you all the way to Longbridge on the south end of Birmingham.
You then have the mighty Lickey Hills to encounter. Difficult to avoid. The best way is possibly via Barnt Green. Or a massive detour via Henley in Arden, but that will add considerably to your distance.
DON'T attempt Rose Hill. It isn't that steep or long, but from the moment you exit the roundabout by The Oak Tree pub (probably called something else by the time you get there) it is a steadily increasing gradient to the top.