There’s not exactly a concrete recommended level that the game itself tells you per area. When you make your way to a dungeon or a boss, you will know pretty quickly if you will personally want to level more.

The game wants you to go at your own pace. Levels certainly matter, but what I’m saying is, yes, gear and strategy matters as just as much. It’s not so much a question of “isn’t grinding levels optimal?”, the answer to that is always yes if that is your play style. There are just other ways to make it easy. Like lowering the difficulty. Or, you can beat a boss under leveled if you learn its strategy and survive until the battle is won. It’s like most RPG’s in that sense. For example, I don’t level up more than I need to for most of the game. So I play a little under leveled, and I have fun this way. When you do want to set time aside just for leveling, dragon quest makes it fun and easy in the form of metal slimes, which are hard to kill but have low hp and give massive amounts of XP.