I don't think ev-training needs to be changed that much, it was already made a lot simpler in gen 6 with the mini games, and s.o.s chaining in gen 7. You should have to do at least something for your Pokemon to be perfectly ev-trained, rather than just instantly feed them unlimited protein and iron. So personally I would rather keep the old limit to how many stat-boosting vitamins can be used on a pokemon and still take effect. Since Gen 8 is going to remove the cap then I think supplements at least should not be sold in regular pokemarts anymore, that would be like being able to get as much rare candy as you want to make your whole team level 100 without doing any battle. Protein, zinc, carbos etc should have to be found in the overworld similar to an ether, or maybe purchasable for a significant amount of battle points in the post game facilities like the ability capsule so it isn't abused.
I'd like to have more information about mints. The full article and the Sword and Shield website implicitly stated that "Using a Mint on a Pokémon seems to change these stat-growth patterns! However, a Pokémon's Nature itself won't change even if you use a Mint." I don't know how I would feel about a pokemon with say a docile or gentle nature you caught in the wild becoming adamant or jolly after popping a mint-- that would be pretty broken. However, it sounds like this isn't exactly what happens. Maybe the mint changes the growth pattern by increasing and decreasing certain EVs earned like less sp attack from battles and extra speed evs, or perhaps it's a change to stats in certain categories that's temporary more like when a battle item is used such as X Defense. So I think this could be an interesting mechanic depending on how it's implemented, but am just not sure yet, and would love more detail. Just as long as you don't instantly get effectively any nature you want just from 1 mint, that would be too much.