I am in same situation. Glad you started this topic just in time. Have you checked out Digital Fabrication for Designers: CNC Router Rotary Axis ? I got the same rotary setup on aliexpress.
I am hoping to get a t-slot plate and mount the rotary setup on it and cut half an inch or so into my spoil board and place the plate in there and remove it when not in use and fill that space for normal operations. Hoping to find the t-slot plate with the correct spacing as the rotary base mounts.
Just wondering how the homing of the 4th axis is done, manually or can it be setup with sensors?
Thanks and looking forward for suggestions, info and inputs.
Be Bliss.