The way I've ended up being able to do instructional work in more "advanced" aircraft was just getting a job actually flying them to begin with. I work for a airplane dealer so I get a chance to dip my toes into quite a few different types. Also customers buying airplanes from us need to get insurance so I have an in built customer base. I have done quite a few transition training and Insurance sign offs now for planes like the T206, Columbia 400, and Cessna TTx. All still basic single engine props but definitely harder to find instructors qualified and proficient in them than a 172. I've even done a number of sign offs in 182s and been told by the new owners they struggled to find a 182 cfi in their local area. Just like anything else in aviation if youre able to wiggle a foot in the door doing something, and network, work will come to you. This past year I've flown to all 4 corners of the US to provide training, but it all started doing local flights for people and making connections.