So. I tried out the new cadence today that was recommended.

It was easier on the legs!

At about 10.3 MPH (was tracking this time) I was in 48/28. I'm pretty sure I was going well faster than 9 mph in 48/12, given my speed in 48/28. My estimation was way off, sorry. I was probably at about 12-15mph then.

Given maintaining one cadence and my lower top speed in town, I could see a 1x10 being more than enough at the moment.

I can really see the logic of a double if one's cadence is going to be both fast and stay tight, but a 21 speed effectively only has about 12 gears. For flatland, low speed purposes, I still think a 1x10 is right. I just need to work with my current gears to see if I should go down to maybe a 42t front.