You're not looking in the right places. I've had great experiences with EmbeddedTS, and their kernel seems to be close to upstream.
This is their latest "High performance industrial grade embedded computer" that I can see on their website
Of course you'll have good upstream support for something as old as the i.MX6UL, it's been out for 8 years (7 years at the time they released the TS-7250-V3).
No wonder Raspberry Pi has upended the market, when a single core Cortex-A7 is the competition.
Even an Allwinner chip that has a terrible BSP at release has good upstream support 7 years later.