Yeah, I'd go hardtail MTB frame and a really good 100mm air fork with lock-out. The fork should weigh in around 1580 grams.
Two wheel sets. Bar of choice.
If you spend time on the big wheel when riding these events, I'd do chainwheels that worked for both. Maybe keep a 3x8 with 11-34 out back and 22-34-48 up front.
Since you are trail riding, you don't need DH speed capable disc's and that will save some weight. A lot of hardtail MTB frames have lugs for both brakes, so you can do more with wheel options ... Also make it easier to acquire a somewhat older frame and build as you wish
Something with 1-1/8" steer will give you tons of light weight options for stem rise & reach and bars sizes.