I'll share a few suggestions that I am familiar with. At my largest (250, 5'10") I was in 18-20W. Now, just on the high end of "missy" (thanks cancer!) I can feel your heartbreak over this. When I see a brand sizing their XL as "12" (or even "10") I just have to laugh (or otherwise I'd cry). I guess my money is just not green enough for them.
You may well be aware of these already:
Lafayette 148 - though it looks like they have ditched Plus, and now go up to straight 18. Still, great quality and generous fit.
Poetry - kind of Eileen Fisher-esque with lots of square shapes and linen, but they have silks and wools, long skirts, velvet. Up to 18 and very generously sized. Jeans and tees too. If you can stand linen, their heavier weights are very nice.
Marina Rinaldi - now sells online! I would have absolutely KILLED if they sold online in my bigger days. They use their own sizing system, so check out their chart for your US equivalent. Goes up to US 24 (they call it "33"). Stuff sells quickly. I thought this dress was not too bad. https://us.marinarinaldi.com/p.....tilo-black
Neiman Marcus (online) - varies, sometimes yes, sometimes no.
Ulla Popken - I used to get their catalog in the early 2000's, thought they disappeared, but still online. Not remarkable but got some good work clothes.
Avenue and Woman Within - rather meh fashionwise, but you never know what you can find. Don't know what Lane Bryant is doing now.
I feel that there was a really grand selection back in the 1990's and early 2000's. Jones New York, Dana Buchman, Ellen Tracy had really fantastic larger sizes, some styles in true Plus. Magnins in SF had a great large department, Macy's used to even have stand-alone Plus stores in a couple of malls. Nordstroms plus section was very good too, the saleslady I worked with was a real genius. Seems like so much plus fashion is now bizarre Betty Page pin-up peek-a-boo crap or cheesy materials.