Cooper) said that the mortgage is assumable (there is path that would allow me to remove spouse from mortgage and retain the current rate), but the process can take a while, is subject to fees, and they need to reevaluate my ability to qualify for the mortgage without my spouse.

Quote from @Quincy Jones: I am also looking in Milwaukee, had a real estate agent literally give me a drawn out map of the areas color coded to show the different others that are the best to the worst, I’d start looking in areas that are starting to or in the process of being gentrified.  

Make sure it explicitly explains the process for termination if you are unhappy with their services, especially if they violate the terms of your agreement.3.

I made quite a bit on it and it was a complete success.

To me, the most important contact is your contractor in the BRRRR process, so I would start by securing a good 1-2 contractors.

Any pitfalls to avoid or tips for making it a success?

He did a full renovation and in the process switched from a wall heater/central forced air to mini splits.

Typically, a good GC can guide you through the entire process and has architects that tend to partner with on most projects. 

Is anyone in the Cleveland area finding success with ADU on property?

I've done a lot of this and can attest to its success (when you know how).