Thanks for your guys concern but before this gets too far I should share that I have been riding for decades and believe it or not I do know how to use a quick release. New bike (for me) rookie mistakes happen. Weight is not everything. Agree with PBL450. If I was concerned about a few pounds, my ass would be the first place to look. I have some light carbon but my steel bikes are usually the ones that get chosen for anything less than a hilly century.
Those top of the line steel bikes handled so well on the flats. Like you say, not too stiff, not to flexy. They'd absorb shocks but not give up the connection to the road. Consequently, they also climb very well, despite the weight. And as you point out, it's mostly plopped on the saddle. 21 pounds is light. It'll weigh nothing when you're up off the saddle.
The lighter bikes I've ridden always seem to steal more energy on climbs. They can't handle the weight moving around on top. Designers keep trying to eliminate that. So they made frames really stiff with fat tubes, put on big diameter press-fit BBs so they don't come loose, and compensated for the harsh ride with elastomer shocks atop the seat tubes and lots of gears in back, and now we got dropper seat posts. Notice they're going back to threaded BBs, pencil thin seat stays, and skinny mainframes?
GCN did a test comparing old steel to modern carbon and discovered only a couple of watts difference on the same climb. Some of that difference would surely be attributed to gearing, climbing in 42-21 vs 39-28. A slight weight handicap would add up on a competitive group ride, but riding solo, great handling would make up for the difference, seems to me.