I've been looking to get a fast telephoto (Nikon) without breaking the bank and as everyone slowly moves to the new mirrorless platforms the prices for DSLR lenses are 30% of their original

I would assume there wouldn't be too much of a deterioration in optical quality from the use of an FTZ adapter as it's really just a spacer to match the original DSLR focus plane

I would also likely have a teleconverter for more reach as needed in front of the FTZ which I think shouldn't pose any more drawbacks than with the original DSLR

my main question is if there's any major advantages to getting a native mirrorless knowing it's going to cost 3x more and lose half it's value as soon as it's bought

there's currently a local seller listing the 300mm f/2.8G VR II for $2,000 USD (currently $5,500 USD new) which could probably be knocked down a bit further