OP I am in a similar boat Macbook Air M1 with Ultrafine 4k + Ultrafine 5k — got a displayport / USB C cable but no luck. Did you ever find a solution? Have tried with both Dell 6000 and Plugable UD-6950PDZ with no luck, although everything else (ethernet, HDMI, USB) works great on both docks.
Did an adapter or bi-directional cable work for you?
Did an adapter or bi-directional cable work for you?