can I go lower than 20 psi on this setup?

Pressure is an empirical thing. Only you can determine what you can get away with with a particular tire at your weight. Riding technique and terrain also both come into play when you start trying to push pressure to the lowest limit. Even then there is that unexpected "hit" that can cause the tire to bottom out, so don't try to push it too far or you will start putting dings in your rims.

what is you 69er setup? I bought an origin 8 ridgid fork

to try on my Giant XTC 26er I was going with the ZTR 355 29er rim on XTR hub, on the 420 mm fork, (this keeps the geometry pretty well the same as my 80mm Pace fork). I thought that on some of the more climby races, the back wheel at 26" would be helpful for an "older" rider.

"to try on my Giant XTC 26er I was going with the ZTR 355 29er rim on XTR hub, on the 420 mm fork, (this keeps the geometry pretty well the same as my 80mm Pace fork). I thought that on some of the more climby races, the back wheel at 26" would be helpful for an "older" rider."

Older (over 50 anyway) rider here too. Mine is the stock Trek 69er full suspension. Fox F29 fork 100 mm and RP2 for the rear. Bonty Race Lite TLR rims. I have had it now for about 1.5 yrs and still love it.

The only downside is that the 26" rear seems to bounce over stuff that the 29" front rolls over. The RP2 helps to even that out somewhat though. Wish I could fit a 650B rim and tire in the back. I feel like that would be the perfect combo with the 29" front.