-TOS had reactors in the nacelles which used dilithium as a mediator. Power was fed down and put into usable energy by other dilithium crystals.
-Intermix Shaft/Energizers - used LOTS of Dilithium but depleted it very slowly under all load levels. There's an initial reaction at the bottom of the shaft and more and more reactants are fed up, with dilithium in each transfer segment as well as being released as particles in the shaft to mediate and create the right kind of electrical charge. Some of the segments siphon off into power converting energizers. The whole thing was very complicated and very hard to manufacture - very costly in dilithium. The bigger/longer the Intermix shaft/Energizers, the more power the ship has.
--TNG (or...late movie) - reactor we're used to. Uses little crystal and it decays evenly, which is fine, because it can be recrystalized anyway (in TNG). Reaction is mediated by dilithium which causes plasma to energize. Step-down power is created somewhere other than main engineering.
I think this works pretty well.
Generally, I feel that the trend from ENT to TOS to movies to TNG works nicely if you assume that ENT an initially overly complex de-centralized multi-component setup, boiling down through TOS and TMP to be more interconnected, and finally arriving at a simplified, centralized 'constrictor core' TNG-type arrangement. Indeed, it would seem that those constrictor segments would serve to eliminate the need for multiple dilithium emplacements - focusing the matter and antimatter streams for a more powerful single reaction.
I believe Rick Sternbach once suggested that VGR's 'swirl core' had dilithium lining it and that it was supposed to be analagous to the TMP core design - so in that regard the VGR core might be a 'throwback' design intended for higher output for the small ship. If one takes lighting effects into account, the Defiant
might have an experimental hybrid swirl/TNG core - with the constriction segments of TNG and the 'swirl core' effect in the M/ARC.
(Of course, for all we know other ships retained the 'swirl' type core as seen on VGR, too. Heck, the Enterprise-D
might have been an exception rather than a rule. Alternately, the)
By TNG, Impulse Engines were powerful enough in their own right the challenge (and risk) of running a direct-connect between the intermix chamber and the Impulse Engines that it wasn't often done.
I agree, and I think the supposed introduction of the modern impulse driver coil (which allows low-level warp-type spatial distortion that makes the ship easier to move at impulse) on the Ambassador
class, which also happens to not have a deflection crystal at the back of its saucer, nicely fits this scenario.