Have you tried disabling traction/spin control? On our 2010 XC90 I've noticed that with it disabled it ensures constant tire spin which is counter intuitive in how you'd think it interacts with AWD and selective wheel braking. Volvo recommends disabling it in slippery or low traction conditions. In SPA vehicles it's the Sport Mode option. In my experience it's helped in cross axle situations and even let me climb on two diagonally opposed wheels with minimal drama. On our 2017 I've not tried it and in Off-Road I've not had the difficulty you seem to comment about.
BTW, I also just saw your Nov 2019 video about drag speeds and one thought I have about your improved performance when the battery charge is lower is that you may be using the often forgotten 3rd motor (ISG) to provide additional torque/power fill. Not sure if it would explain it as it should be engaging as a generator and creating drag, but if you had enough power it would instead be providing more. I recall having read a technical article that mentioned details from a Volvo powertrain engineer. The article mentioned how the torque fill function and added power was good for a theoretical 46hp and that Volvo didn't include it in it's total spec given it's an on demand delivery. If you do any more runs, perhaps you could have the Driver Performance up on Sensus and see if you notice the ISG engaging during any of your runs to help explain the additional boost. Something else is having a OBDII reader checking the battery voltage. When fully charged it should be delivering 400V and at 25% significantly less (not sure how much) and that could be driving the computer to engage the ISG for supplemental fill significantly offsetting the lower battery power.
Plus they didn't want to call the T8 a "tri-engine". It also somewhat jokingly mentioned how if they had they didn't want it to be compared to the Ford Tri-Motor. Made me laugh since early in the XC90 T8 forum there was a whole "Twin Engine Gate" thread where people were raising pitchforks insisting Volvo was crazy mad for calling the powertrain Twin
Engine.
Found the article that I was recalling:
https://www.sae.org/news/2016/02/volvos-new-plug-in-xc90-is-really-a-tri-motor
and reminded me of this post. This one was easily 15% climb over hard crusty more than 12" deep in parts and the start of the climb over the white stuff was cross axle. Last few days we've encountered other paths with steep cross axle climbs and they were pretty easy to tackle with proper placement. 
