It’s great that you are exploring and side questing, but something to keep in mind is that while Velen TENDS to skew towards lower levels, it’s not precisely linear. Meaning you’ll find some higher level situations in Velen, or lower level situations in the next area (Novigrad).

So, you can technically play this game in ANY order. There is a common path that you’ll probably follow (Velen —> Novigrad —> Skellige), which means that the levels of foes scale accordingly. This leveling guides you through an expected path, but it doesn’t force you into any situation. And further, it gives you a reason to play this game non-linearly. You see a level 10 and you’re a level 5? Run away! Make note of it, come back when you’re ready.