So I did a ton more research guided by the information you all gave me. Thank you 🙏
I just pulled the trigger. I'm getting a The Casper King from Casper through Costco Online. It was $800 on Costco as opposed to $1200 on Amazon/Casper direct. The best part: the return policy. Say it sags in a year, like some report, I'm still able to return it. And I can bring it down to Costco in person. No need to painfully compress it into a box. Casper's 100-night guarantee is limited to just that: 100 days.
I was indifferent about the brand. It's almost a toss-up between Purple, Leesa, and Casper, but the Casper was the only one hundreds of dollars off through Costco. They all have nearly identical foams, so it's all about branding. I would have bought the Leesa if it were $800, because the WireCutter (which has never led me astray) claims that it's slightly more supportive for stomach sleepers like myself.
I almost went for a Zinus memory foam 12" mattress because the credible reviews were better that more expensive mattresses, it's a fraction of the price at $352 (king), and Sleep Like the Dead had decent reviews. But I became skeptical because its 21k 4.2-star average Amazon reviews apply to all product variants: 4 thicknesses and 8 sizes. People could be reviewing a mattress that their kid sleeps on. And a lot of reviews are for the thin 6" mattresses that go in college dorms and vacation rentals.