OP
I think y'all are right, we should just stick with soft plates. After reading and experimenting some more, I think I'm cured of my metal backplate FOMO. Incidentally, the softplate+harness I already have is actually the most perfect for the wing I already have, dare I say even better than their own brands' products. And both are among the lightest pieces in their categories. The OMS IQ Lite harness has a few specific unusual quirks that incidentally allow extremely solid pairing with the Aqualung Outlaw wings, eliminating the (minor) tank tilting and rolling problem. I'd choose this over the proprietary Aqualung Outlaw harness.
So I'll just give my buddy my entire set. She can be my kid's twin. Now it's time for me to diy my Frankenstein tropical single tank sidemount kit. I'm thinking DECO 22# wing + Mares XR Soft Backplate + Aliexpress comfort adjustable harness with curved shoulder pads, hip pads, and pivoting shoulder straps xD it's a bit heavy (1.16kg) but it has all of the curves and rotation my boobs would appreciate. I find the straight down shoulder straps style to be very uncomfortable . The Mares soft plate looks sturdier than the OMS IQ Lite so I'm guessing it might hold the tank on the side with loop bungee a bit better.
