By the time the CGI version comes around, though, you start seeing more Galaxy
class features. Since those ships also have much higher registry numbers than the initial TNG batch, it might be safe to assume that at some point the design itself got updated with Galaxy
style improvements for things like sensor bays and the deflector.
My guess is that Starfleet never expected to be able to produce enough Galaxy
class ships to completely fill its needs, and faced with the complexity of its development, decided to take the initial design items and start fabricating them in advance of Galaxy
production, leading to the slightly older looking Nebula
class ships during TNG. By the time the CGI Bonchune
model rolls around, Galaxy
production has been restarted and the design is well established, so they started building ships with common components to increase interoperability.
Also the recent novel Peaceable Kingdoms
notes a Nebula
-class ship is 30 years old circa 2385, implying that the class saw production almost a decade before the first production Galaxy