I've never visited the heart of the Dolomites, but I'm sure it must be mind blowingly beautiful if you like rugged limestone peaks towering above scree slopes, lush green meadows, cold shady forests and good food. Round trip like:
Bolzano - Predazzo - Alleghe - Cortina d'Ampezzo - Stern - St. Ulrich - Bolzano.
(Something I've been considering for a bike holiday with my girlfriend).
Unfortunately it's not a point to point route
Seriously, Marmolada, Sella group, Tofana, Tre Cime di Laveredo, Civetta, I'd probably just build a route along/around those mountains.
Remember, you have a bike, you're in a highly developed place - bikepacking doesn't get much easier. If you want the freedom to sleep where ever you feel like stopping (on top with great views...), but not bring stuff for boiling water/food - there's huts all over the place. If the weather's poor show - huts all over the place. You basically just need a thin, little mat and quilt/sleeping bag in addition to what you'd bring along anyways. September will be great. One of the best times of the year for such regions and such activities.
I do a lot of tech riding with luggage and it's quite alright. I may be more tired at the end of the day and perhaps struggle a bit earlier in difficult stuff, but sleeping system (perhaps a thin set of longjohns/long sleeve base, socks, buff) and even a very compact kitchen will add about 1 kg (at least in my case it would).
If sleeping out is an important part of your riding and you think that trail biking doesn't go well with bikepacking - give it a try... you'll see, they go hand in hand really well (if you're gear is compact and light).
If you'd like to taste more of the Western Alps, where things are high and big, Val d'Aosta may be a place. Mountainbiking doesn't get much bigger than there. You'll struggle to find single tracks with a comparable amount of vert loss/gain no matter where on the planet you are.
In the Alps' main ranges I struggle to develop and ride routes where I get less than 4500 m vert gain and loss on 100 km distance. To be honest, that's already trying hard to keep it "flat".
The Dolomites are a bit more mellow and thanks to the wars, loads of the uphills can also be ridden - should be fun:-bd