yeah those kind of fillets will never be clean without supports. You could maybe use a different material for the support but that will increase the print time drastically.
The easiest and the best approach in my opinion, is to rework the geometry of the model in ordre to avoid the need of support at the first place.
A good practice is to make a chamfer instead of a fillet. And if you want to smooth the design, you can add a fillet on the top edge of the chamfer.
Here, I’ve made a quick model to show you And I’ve played with the 3mf