Simply removing/replacing some units from ITJ and UV could create a Standard difficulty. Mechanics can stay the same. I'd be in favor of that.
I usually prefer things to be accessible and lenient since, as a non-profit volunteer project, we have the luxury of not having to care about gating content behind a paywall or haggling for every ounce of retention and engagement. I'm against introducing unnecessary grind. Too many games on the market court for endless player attention, it gets grating and exhausting. But opening up season challenges to be done on Standard seems good. Players that can will skip to the hardest set-up anyways, so they are not impacted. Meanwhile, everyone can participate and get some points. Although, while I'd personally like the Promo Sunderer to be accessible with like 60-70% effort, it won't be in reach by only doing easy versions of challenges.