Gen II provides a ton of desperately needed QoL improvements to Gen 1. After Gen 2, though, you kind of start getting as many annoying changes as you do positive QoL improvements – e.g. balltism, the tedium of EV training, a 50% chance of having the wrong ability (or a 100% chance of not having its HA), a >50% chance of having the wrong nature, Egg Moves become must-haves instead of fun gimmick side quests... the only thing that's been added from Gen 3-8 that Crystal is missing is the Physical/Special split and honestly.. I don't think I'd make that trade.
Crystal is the only game I've been replaying since back when LGPE was revealed and the franchise was obviously dead.