I like the addition of research levels as a mechanic since it makes you engage a little more with each pokemon to fill its entry out instead of tossing a ball and moving on and you get a little shiny bonus for going completionist on a mon, but what changes would you want made to the system for a DLC/second Legends game?
I would lower the amount of mons you have to capture for completion to something more like 15 or 20 and make more bonuses tied to observing unique behaviors that usually only get mentioned in the pokedex. For example there could be research tied to stealthing into Zangoose territory and witnessing them get into fights with Sevipers, or watching a Magikarp swim up a waterfall to become a Gyarados. For the sake of making these events a little more based on player initiative maybe you would have to lead the Seviper into the Zangeese by throwing food lures to get them close enough to eachother, or the Magikarp only becomes strong enough to climb the waterfall if you throw vitamins for it into the water. Could even implement a little Pokemon Snap and give the player a camera so they get the credit for these research moments by snapping photos as it happens.
Secondly, I think there should be a fex extra rewards in between the level that gives you the pokedex entry and max completion, to incentivize furthering your research level even if you don't want to 100% every last one. Maybe like unlocking extra moves or stat boosts to the species as your understanding of them grows or something?