I think we have similar tastes, @GlitteringUnicorn - I love The Fold and Cefinn as well. I'm a bit shorter than you and a size 16, so apologies if my suggestions are wide of the mark!
If you want French chic, you could look at Tara Jarmon - lots of plain/block colours, but a little bit quirky.
I have no idea whether it's 'cool' or not, but I like some Gerald Darel pieces as well - a lot of this season's dresses seem to be quite 'busy', but Vinted has plenty of more classic dresses.
I think Winser London are on hiatus (or they may have stopped trading - I can't tell from their website), but there are plenty of Winser dresses on Vinted which you could check out?
I find that Jigsaw dresses suit my body type, but I realise you're taller and thinner, so your mileage may vary. Some of their prints are busy/floral but not all. They use a lot of viscose, but that suits my lifestyle as it means the dresses travel well and rarely need ironing. I've just looked at their website and am a bit bemused by the dress selection - lots of sleeveless styles I'd be freezing in now, let alone in a couple of months - but presumably they'll release more autumnal clothes soon, and I got a lovely Jigsaw work dress for £8 on Vinted recently...
LK Bennett is usually a good option for decent-quality, plain dresses which hang well. Ghost (the original brand, not the M&S diffusion line) is a good bet if you want fairly simple colours and shapes and materials which feel nice.