The 10 seconds maximum transition time is indeed not really suited for making very slow fades.
What you can do would be either assigning the upper layer to crossfader and move the crossfader manually with a midi controller, or a lighting board via Artnet,
or create duplicates of the clips in the same layer,
animate the opacity of the first copy of the clip to timeline, and forwards, one shot, set the speed to how fast you'd like your clip to fade in:
and the copy of the clip - to make the fade out, - opacity set to backwards, one shot, and the clip to continue to play from the previous clip's position, and set the opacity speed how fast you'd like your clip to fade out: