There are many places to escape the heat but hard to find 55+ places.
Our favorite areas would be southwestern Colorado, the Oregon coast and the Olympic Peninsula of Washington, McCall, Idaho, Stanley, Idaho, Coeur D'Alene, Idaho.
Closer to you in Texas would be the White Mountains of Arizona - yes, Arizona has areas of cool temps in summer
(Show Low/Pinetop); Flagstaff, AZ or Ruidoso, New Mexico, Red River, New Mexico; Eagle Nest, New Mexico
Having lived in Michigan I don't consider it to be overall cool. Yes, there will be some cool days but not consistent.