Having grown up in Detroit and making annual trips to visit grandparents in Hernando County, then living in Orlando and driving back home to family in Detroit, my recommended for getting to Orlando is I-75.
I-75 crosses Kentucky at elevations comparable to those in northern Ohio, it is just that there are valleys to cross in Kentucky so the road is not so flat. But it is not really different than the road from Louisville to Nashville. From Tennessee state line to Knoxville I-75 crosses a few low ridges then runs in a valley down to East Ridge (east of Chattanooga) which is the only place were you'll find some longer 6% grades, a few miles rather than a half mile or so.
You can go through Nashville, but the terrain to Knoxville and to Nashville are pretty much the same whether you go through southern Indiana or southern Ohio and Kentucky. From Nashville, I-65 doesn't go to Orlando, so you would be taking I-24 to Chattanooga, and that crosses more ridges than I-75 coming from Knoxville.
You can totally avoid significant hilly terrain by taking I-75 down to I-80/90, then I-80 to I-55 coming out of Chicago (Illinois flatlands to Mississippi River valley), then I-12/I-10 across the Gulf coastal plain. Don't take I-94 across Michigan, because that's going to have some hills crossing glacial features in SW Michigan that are just as formidable as what you encounter crossing southern Ohio or Southern Indiana and the valleys in Kentucky.
