Well, after lots of research, I finally acquired a new bike and wanted to follow up with my experience with it. I settled on a Cannondale Scalpel HT 4, because it combines the light weight of a race bike with a head angle closer to a trail bike, and managed to find one in stock at a bike shop that could ship to me (my local shop said they wouldn't have one for 11 months). I did two easier test rides, then took it on a longer trail that was at the edge of my abilities on the old bike. I won't say this is multiples better or changes the way I can ride, but it makes things maybe 10-20% easier, and that was enough to allow me to complete the whole route without putting a foot down.

Specifically, the 29" wheels do indeed feel more capable over rocks and roots (and feel much faster when I ride a paved road to the trailhead). The slacker head angle and longer wheelbase also give a lot of stability, and I haven't noticed any loss of agility. In fact this seems to handle switchbacks better somehow -- I can really crank the handlebars to one side and still feel stable. The 760 mm handlebar width felt natural, although I need to dial in the position to improve the wrist angle -- but when I jumped back on the old bike for a comparison, its 560 mm handlebars suddenly felt ridiculous. I've felt the most benefit on rough or loose climbs. While the old bike would be wheelieing or spinning out, this one just churns uphill. It feels like a tank -- but a svelte, 24 pound tank. And then on smooth climbs, it goes up like a road bike. The 1x drivetrain is super convenient, and the 100 mm front travel compared to 60 mm on the old bike has been nice on rough descents. My position doesn't feel squished as I feared, it just feels natural when riding on trails.

I still have skills to work on (now that I'm riding over more stuff, my new nemesis is pedal strikes) but this does make the chunkier trails more rideable for me. Thanks for encouraging me to make the leap.