Low pressure (5-10psi) was initially for riding in sand. On the trail, I started at 20psi, but it was too bouncy in many sections and I lowered it to about 15psi. 20psi seems great for pavement but I don't really ride it on pavement.

What I've noticed is actual diagonal 'stress' lines on the sidewalls that were not there when I bought the bike. Bike was used but had been stored unused in an air-conditioned storage unit for 4 years. So I'm wondering if the tires (or at least the sidewalls) have become slightly brittle from age?

These diagonal lines were not there when I bought it. They are around most of both tires. Since I'm converting it to tubeless, I suppose I'll find out if this is an issue caused by the tubes soon enough.