I registered to the forum just to reply to this post.
I am a retired Mechanical Engineer with a 27 year career at Cessna. Although I had ratings through Commercial/IR/AMEL I never made the jump to military (4 years in USAF) or professional flying due to real bad uncorrected vision and strict standards at the time.
Cessna was a good career choice for me. Depending on your position (I worked turboprop and and jet engine installations and fuel systems) there is a lot of opportunity to interact with Flight Test and ride along and observe flight tests. Other positions such as structures might have less opportunity. Flying opportunities include service tests, a good flying club and a limited time flying company transportation in a 414A. The higher up the food chain you go, the less opportunity. I never even flew right seat in a Citation.
You mentioned Flight Test Engineering. Some Flight Test engineers fly right seat on their project, with opportunity to to become type rated or progress to Flight Test Pilot. Others have more analytical jobs. I once unsuccessfully tried to transfer to a Flight Test Engineering position. I think having direct flight test experience and possibly an Aeronautical instead of Mechanical degree might help, however, there is a lot of competition for a limited number of positions. Long hours sometimes, a lot of paperwork, and keeping track of such things as maintenance, schedules, instrumentation, etc.