There is a manual workaround for changing the bush planes in the config files for each bush trip.
https://forums.flightsimulator.com/t/how-to-change-aircrafts-of-bushtrips/211059
It’s not super convenient but it works. I switched out a stock MSFS plane for the third party AT simulations Piaggio with that method and it work.
Another option is to select developer mode. Then switch the plane while sitting on the runway during your bush trip. I haven’t tried this method but according to someone on the forums it works.
Hope this helps.
PS: and yes I agree. It would be awesome if the sim had an option to change the bush planes built in.