I don’t exactly disagree here, but personally I would contend that even the old Mythicals are, for the most part, not particularly well-developed. Deoxys is no less “one sentence worth of lore” than Volcanion, the only difference is that (assuming you had the means to attend a special distribution event) you could go catch it yourself on a dull little island.
I think a lot of peoples’ impressions of the lore of older Mythical Pokémon is a lot more informed by the movies than they think (well, the movies and playground rumors), so in my opinion “it was designed for a movie” isn’t necessarily an issue, even though the in-game implementation the Gen 6-8 Mythicals could definitely have been better.
I feel like you can actually narrow down the period in which Game Freak were doing a semi-decent job with Mythical Pokémon to a very precise 2008-2012. That’s when they were both much more widely available for the audience (as opposed to “Hope you can afford to fly to New York!”) and implemented with bespoke in-game events that, while not astonishing, at least said/portrayed something about them in a unique location. This was also the period where I feel they put a little more emphasis on alluding to the presence of these Mythicals prior to their distributions, like how they did with Mew, which I think helps to build up anticipation.