I have a plated CRF250RX and a CRF450L. I think something like the 450L is really good for what the OP described. Yeah, brand new it is pretty expensive, and it certainly is a lot better with an aftermarket ECU. But if you can find one for the right price, used, already set up, it's a great bike. I ride one at Zars Ranch and have fun with it there. That being said, it's a lot harder to ride in tight single track than the CRF250RX I usually ride at Zars. SHNF is not as tight as the trails at Zars, mostly double track, and sandy with tons of whoops (if I remember correctly). I think a CRF450L would be great out there. It's awesome for the perimeter around Zars where it gets sandy like SHNF in a few places and good in the sand pit. It's a sand whoop eating monster.
I've recently had the opportunity to ride a few Betas, namely the XTrainer, 200 RR Race Edition, and 300 RR Race Edition. Now I want a Beta. They are easier to ride off road than anything I've ever tried, absolutely perfect for the single track at Zars. If you get one in San Marcos, they will get it plated for you; at least that's what I've heard from those who have purchased there. I don't know how a plated 2-stroke would be for a 40-minute ride to get to the trail, though. I know my plated 250RX is ONLY good on pavement to connect trail, with short connections only, not more than just a few miles. I've not ridden a Beta 4-stroke, but if the 350 or 390 handle anything at all like their 2-strokes, I think one of those would be about as close to perfect as you can currently get for that sort of thing; lighter and easier in tight stuff than a 450+, but with the legs for road-going better than a 250 four-stroke or any two-stroke. I'm considering getting a 390 RR-S to replace both my CRF250RX and my CRF450L. But that's a tall order. I like my CRF450L so much, I'll probably keep it no matter what. It's just too good and too versatile. I'd gladly trade my 250RX for any of the Beta two-strokes I've tried.