I don't have any suggestions but just wanted to say that I commiserate.
I do own one belted trench, which is black and made of "packable" material (very lightweight nylon - no lining), from BR several years ago. Even if it's not warm at all, since it has no lining, it looks fantastic, so I keep telling myself it's all in the cut and fit. But alas this is the second spring in a row I've tried on a gazillion belted trenches -with lining for warmth- and can't find a single one that flatters. I look thick and frumpy.
I am now thinking my ideal would be grey.... which only complicates matters more. (The right white was hard enough).
Also even if I found one with a belt, I'm not sure I *want* a belt. I find they are a hassle and I end up tying them in the back to make sure they don't drag on the ground. But worse than the belt are the darn epaulets and the buckly nonsense on the sleeves. Sigh...