You know something that often goes under appreciated about DP is that it lets you explore most of Sinnoh with only 2 badges.
I like how after you get the bike from Eterna City you immediately unlock like 9 routes, 4 cities/towns, and a bunch of optional side areas. It feels like the perfect point for the game to give the player some downtime to just explore the game at their pace.
Of course, there are still a decent amount of areas between each gym after the fourth, they're just tucked away in the various edges of the map. For the most part you can see like 70% of the region and most of the major landmarks with only 2 gym badges. Which is convenient because that also means most pokemon in the dex (with a few exceptions) can be caught before most of the major battles, making them especially fun to use in these games.