I don't weigh 325 lbs but when I started last year, I was above 285, and I ride steel. it didn't feel flexy or noodly to me, but, YMMV. It is a personal preference, and I have tried aluminum bikes (though not aluminum road bikes), and found the ride harsh compared to steel. When things are dialed in on a steel bike, it just feels like you can ride as far as your legs, heart, and lungs will take you. That said, there are aluminum bikes with comfort features built into them, like the Cannondale Synapse, and Trek Domane series. Worth checking out as well. I have no personal experience with those so I cannot say.

My suggestion is, since OP already has two aluminum Treks, try a steel road bike that is not a Trek. Nothing against Trek. They make fine products. But there are other brands.

Suggestions for steel road, or touring (or touring ish bikes) would be
Jamis Bossanova (touring ish bike)
Jamis Aurora (touring bike)
Jamis Satelitte Comp (road bike)
All City Spacehorse (touring ish bike)
All City Mr. Pink (Road Bike)
Surly Pacer (Road bike)
Surly Long Haul Trucker (touring bike)