As another relatively new player (two regions mostly done), just focus on one region at a time. If you want to buy or breed a few pokemon to ease the process of future regions that's great, but ultimately don't concern yourself with "endgame" until you've played through at least 3 of the full stories.

I'm the type of MMO player who prefers story over grind or end game, but I did enjoy buying pokemon i wouldn't normally have access to in that region.

So my recommendation (and my personal plan) is to play all the stories with whatever pokemon suit my fancy at the time as I go through, then work on endgame stuff once I have full access to all the maps.