Clearprop
Whilst most of what is above is exactly spot on, It is possible to get there through dogged determination and a modicum of right place right time serendipity.
I now work as a full time Developmental TP in General Aviation, for a manufacturer, and it's only taken about 25 years to get there.
I started as a display and demo pilot doing contract ad hoc, then full time as production TP. A new company product led to an offer of developmental/ experimental TP work under tutelage of qualified FTE. Short course at NTPS ( great people as referenced above) and refereshers at regular intervals.
Almost continuous work on upgrades and newer models FAR 23 at latest amendments,and now in middle of FAR 23.221 campaign for 10 seat turbo prop.
Every day is a new day of wonder. As an Agricultural pilot 30 years ago, having missed Air Force selection due to wearing eyeglasses, I never expected to be strapping into another new type and working through a full part 23 certification program at this time in my life, but wow it has been worth the effort and unfortunate marital ' restructuring' that was part of the deal.
It's not AB or Boeing by a long chalk, but it's amazing and rewarding work every day.
I travel the worl extensively and have flown in about 30 countries thus far as part of the job. I have about 1500 FT hours and am on an ongoing learning program.
Someeople tell me how lucky I am, but 30 years of hard work has helped to make me lucky indeed.
Cheers and all the best in your quest.
HD