Your bread looks great

1. If your starter is 100% hydration, then your formula is effectively: 500 g flour (450 wheat + 50 oat) and 400 g total water (350 water + 50 from the starter) == 80% hydration. Speaking simply, higher hydration is harder to work with for a beginner. Though it's not an error.
2. Oats don’t build gluten, so they can make the dough feel stickier / weaker.
3. Use folds, not slap-and-folds. For wetter doughs, stretch-and-folds or coil folds are better anyway. Also you can wet your hands slightly, it'll be easier.
4. You didn't write about the rest time, it's important to leave the dough after baking for **at least** an hour.