DogBoy

I'm going to give you a bunch of unsolicited route advice.

If I were to do this ride I'd use the elory-sparta trial system all the way to reedsberg.
From here I'd take roads down to Cottage Grove, and I'd skip Madison (traffic) unless you specifically want to include it. I'd go through Baraboo, then to Devils Lake state park, then take the Merrimac ferry, through Lodi, Dane, Deforest and then south through Sun Prairie to Cottage Grove.

If you get to the Glacial Drumlin Trail (GDT) and want to get off the crushed limestone you can take roads that are fairly light traffic and pretty flat:

take county road BB out of Cottage Grove
Becomes B
When you Get to Cnty Road F (After Johnson Creek) go straight onto Concord Center Dr.
Follow this until it dead ends, then go right on Cnty road DR (aka Delafield rd)
Then take a right onto Golden lake Rd
left at Gennesy lake rd
right at Cnty Z (AKA Dousman Rd)
Follow the fork to the left and go left on US 18 for about .1 miles (there is a shoulder)
Turn Right on Main street and follow it until you get to the GDT. Its paved from here to Waukesha.

I've ridden both the GDT and the route above many times and find that the road route is about an hour faster, but not that scenic.

I don't have any real advice for getting from Waukesha into Milwaukee other than to say the New Berlin Trail sucks. It's gravel and feels more like a power line access road. Getting to Greenfield park isn't bad, but my destination is always West Allis so you are on your own from there.

The GDT ends at the intersection of College and Praire ave. Make your way through Waukesha (I use College Ave to E Racine to S. Porter Ave to E. Broadway but there are places where college doesn't feel that comfortable) until you get to E Broadway. Follow this until it becomes Cleavleand Ave and then go N on 124th and you will be at Greenfield Park. You can do the Oak leaf trail or use a route you already know from there.

Have a good trip and sorry for all the detail.