Anyone want to share their experiences with them? I'm a big guy (6'3", ~270), so wondering about longevity and frame flex. Cheers.

The group set has nothing to do with frame flex, for what it's worth.

Anyways, the tubing is Unbranded Reynolds 520 that is contracted for them by the same tubing company which has the rights to make Reynolds 520. This has been confirmed by the guy who started All City.

Basically, it's conservatively butted OS tubing that should last for decades of normal use.
There may be some flex in the bb shell area due to rider and luggage weight, but nobody here will know for sure. Flex there isn't inherently bad and if you spin more than mash, twisting is greatly reduced.

The Advent X shifting is high quality overall. It's been around for 4 or 5 years and is common.

If you have access to a bike shop that allows test rides, try a comparable bike with an aluminum frame. During the first year Cannondale bikes were available, I sold one to a 270-ish pound bodybuilder who had complained that every steel bike he'd ridden felt like overcooked spaghetti. I had him test ride the Cannondale, and he bought it on the spot.

I can't comment on frame flex...not big enough, nor strong enough to flex a frame. I do have close to 5,000 miles on a 1st generation Spacehorse frame though. I've used it primarily for general road/gravel road riding, with the occasional tour. My tours are relatively short, and typically do not have more than 25 pounds of bags and gear on the bike. I've tried both front loading (I though the steering got a little unwieldy), and rear rack loading. I tend to prefer keeping the weight a little lower in front, but that's largely personal preference.

It's been a solid performer, and I have no complaints.