Why not look at brands like Farm Rio or Rixo then? The dresses you've picked aren't a million miles from trad M o B.
If I'm ever MoB then what's suitable for M of B is a dress I fell in love with and makes me feel happy wearing it. It would definitely not be those dresses and my son would think, what on earth happened if I turned up in one of them.
You could try looking at all dresses on Liberty or MyTheresa and see if you find inspiration.
I personally strongly disagree with buying one off "occasion" dresses but there's plenty of brands like Farm Rio, Rixo, Alèmais, Stine Goya, Pink City Prints, which are extremely versatile. And many of these will be poplin, linen or silk.
This is just from a local shop and I've had my eye on it just because I like it.
https://frontiers-woman.com/products/siam-dress-tartan-print-navy-viscose-linen-satin