Candidly as much as I enjoyed FJ's designs he did take the easy way out. He did what a lot of fans were doing and continue to: rearranging existing components. But it's not very creative.
In the real world I really wonder how many major components are interchangeable between a carrier, a cruiser, a destroyer and a transport. I wouldn't think many if any. Certainly the hulls and super structures aren't interchangeable.
The first genuinely divergent Starfleet designs we got were in TAS (the freighters and shuttlecraft) and then next in TMP and TWOK. And note that while there were some shared elements there were also distinct differences which is as it should be.
I also like that some fans have learned to go beyond just rearranging the same components. From that we've gotten some inspired designs, some of which look authentic enough to fit right in alongside with what we've seen onscreen.