Best performance (now likely not noticeable much time) on and off road is to run the smallest rims that will fit over the brakes. Running larger rims than that is approaching, never a performance advantage. Now does it matter in the real world the vast majority of the time no.
Heck, even with formula 1 the car is got slower in every performance metric when they switched to 18 inch rims to give the cars a more modern look.
There's multiple trades and many do favor smaller wheels. To your point, there's additional advantages to moment of inertia, rolling resistance, etc, on top of those advantages already identified. I absolutely agree and there's no argument to those facts. An 18" can make sense for many.
IIRC, you've had Porches. Consider:
2021 GT3: F 9" x 20" - 52, R 12" x 20" x 44
2021 GT3 RS: F 9.5" x 20" - 50, R 12.5" x 21" x 48
The faster more extreme version steps up to a larger wheel. An extreme but it is the same fundamentals. What can't be dismissed is handling and stability advantages which shouldn't be conflated and aren't negated with the pros of smaller wheels you point out. As others have attested, many times over on these boards, the additional compliance with more sidewall. Most times preferred. But just as many times results in surprise. Does it make a difference day to day, perhaps subjective and depends on driving style, but there absolutely is a difference. Especially with really large tires. In my use, I prefer 20s with 35", for the way I use the vehicle. I drive fast on mountain roads, tow heavy, and definitely still want it to play hard in the dirt.
Back to the point of this thread - will 20s and a large 34" tires air down and play well in the dirt - heck yes. And it'll do it with confidence and performance. More importantly, it will retain some measure of better stability and handling where the vehicle is 98% of the time.
