I've watched a few episodes from each season and kept up on the news, but I havent religiously watched it by any means so I might be wrong on a few details.

Every season has followed Ash, but they will be moving on to a new protagonist soon.

It is very loosely connected to the games like it always has been. The two can be enjoyed independently of each other.

Not entirely sure on the story, but Ash did finally become a Pokemon world champion in the most recent season, which is why they are changing the protagonist.

He hasn't caught any legendaries, but he has a mythical Pokemon melmetal.

Based on what I've seen, you can start almost anywhere. Each season softly resets Ash's progress. You may see occasional cameos of past characters, but each season Ash usually makes a new set of friends, has a new set of Pokemon, and starts a new story.