To follow up:
I’m still trying to get this to work, but my lack of knowledge on how U-Boot or Device Trees work give me little to show for so far.
First: Boundary has a newer version of their U-Boot fork at GitHub - boundarydevices/u-boot-imx6 at boundary-v2020.10.
That one fixes the conflict between video output (legacy video driver) and using the device tree driver model (needed for pwm-backlight) from the older version and includes a driver for the SN65DSI83 which might make creating a driver for the SN65DSI86 easier.
I’m still trying to get the backlight to work (dm tree shows the backlight entry I created, but I don’t know how to force it to be probed/enabled) and hope to make progress their over the next days.