Regarding Pokémon distribution and availability in non-remakes past Gen 5, here are my short opinions, I'm not in the mood for writing anything longer.
X/Y: Excellent. The largest regional dex to date, with great variety. All regions are well represented, and I never felt that the Kalos Pokémon themselves were that hard to find. There are none that are downright rare, they are uncommon at best. And IIRC all of them are used on the team of some important trainer, the only exception being Florges. I really liked the diversity of the Kalos dex, it was always exciting to go to a new route to see what I would find there. It made it so much more fun to explore and play through the games. Special encounters were also really great here, always offering something different compared to what you would usually find.
S/M: Awful. Literally the polar opposite of X/Y since the variety was bad, and Kanto was massively overrepresented in the regional dex. I mean
this is absurd. While X/Y had more Kanto Pokémon in terms of raw numbers, it was less in terms of percentage, and to me, it
felt less since the other regions were so strongly represented too. In S/M, it was completely obvious right from the start that the games had a high Kanto bias, the games didn't even bother hiding it. The diversity on the routes and areas is also really bad, with around 10 Pokémon lines being found almost everywhere, while many other Pokémon are rare. Most notably the Alola Pokémon themselves, quite a lot of them are uncommon or downright rare, full details
here. When I play a new Pokémon game, I want to use and encounter
new Pokémon, not have Kanto Pokémon spammed in my face every other encounter. This really bothered me when I played the Alola games. The special encounters were also really poorly handled in Gen 7 since even if you had a special encounter, you would mostly find the same Pokémon as in a regular encounter, which was extremely disappointing. Add in the awful SOS mechanics and the fact that some Pokémon were exclusive to them (at low encounter rates, no less), it became almost unenjoyable in the end. Having 150 less Pokémon than X/Y in the regional dex was also a mistake since it contributed to even less variety. The Island Scan was kind of cool but a bit complicated to use, I never used it during any of my in-game playthroughs.
US/UM: Better than S/M, but worse than X/Y. The Pokédex expansion helped, too bad they didn't fix the variety. Sadly, most of the issues from S/M remain in the end. This seems to be a very unpopular opinion, but I think the Alola games (S/M especially) failed in terms of diversity and showcasing their new Pokémon, while X/Y succeeded.
S/S: Similar to X/Y. Does a fairly good job at showcasing their new Pokémon, but it isn't perfect. Overworld encounters help a lot with encountering the Pokémon you want. The main issue is that some Pokémon are only found in specific parts of the wild area, and some are locked to specific weather conditions. But on the whole, these games had great variety and a solid regional dex, I rarely had any issues finding any of the Pokémon I wanted, new or old.
ScaVio: I actually think these games did it the very best. High variety from all regions, but the new Pokémon still get a lot of the spotlight. Overworld encounters combined with having over 100 new Pokémon helped a lot. It is hard to explain, the variety and distribution just sort of felt right when I played Violet.
Here's how I rank the modern games when it comes to regional dexes, variety and showcasing their new Pokémon: ScaVio > X/Y & S/S > US/UM > S/M