My recommendation would be to increase the insulation of the cake tin....say, a layer of aluminium foil and then four or so layers of baking paper and then a few more layers on the outside of the tin to prevent that burning. Then crank the temperature up a bit higher (not a lot) and try again. If it sags, but doesn't burn, go higher again until you find the sweet spot. The middle usually sags due to the denser centre not being cooked fully and dragging the top down as it settles when cooling. The problem is, going higher results in burning and cracking, unless your insulate the tin appropriately. Hope this helps!
