Old thread revival there...
In my anecdotal experience, sun exposure is the largest contributor to tire sidewall degradation.
When I was a kid, Dad had a '40 Buick that sat parked on the street most of its life because we didn't have enough driveway space, and it didn't get driven much because cloth-covered wire tended to fail and discharge the battery and wreak havoc on the electrical system. The outside sidewalls always looked terrible on it, the inside sidewalls didn't.
On my Jayco single axle travel trailer, the tires manufactured late 2018 had the start of cracking in the sidewalls on the outside by 2024, perfectly good sidewalls on the inside where they were mostly covered by the trailer.
On my new-to-me 20 year old Roadtrek, the original factory spare tire had horrific sidewall cracking where it was exposed to the elements in that gap between the cover and the rear door of the van, but the rest of the tire looked fine.
On my 1995 Impala SS, when I bought it with around 6,000 miles on the odometer in 2013, the original seventeen year old tires had moderate cracking in the sidewalls, more on the outside than the inside, and the car had been garage-kept its whole life. The spare tire that had never been mounted on the car and lived its entire life in the trunk had no cracking whatsoever. The car is up to around 43,000 miles now, still garage kept, and while I have relegated that factory spare tire to the shelf (put another matching Impala SS wheel as a spare) that original spare still has no cracking on it despite being now 31 years old.
Covering the tires while parked is the cheapest effective solution. I would expect that if one feels motivated to put a vehicle up on jackstands, that lowering the tire pressure while up in the air could also be beneficial, but this would be a lot more effort, time spent mothballing and restoring to roadworthiness, and risk depending on the weight and size of the vehicle.