That's quite a tricky dress code! I don't know what the crowd will be but I think you'll see quite a lot of variation amongst guests.
I definitely.think wedding style has moved on from 20yrs ago, when I first started regularly attending them - I'd be unlikely to wear a fitted sheath-type dress now, for example, and that was a bit of a default back then. I tend to navigate towards midi length, with a fuller skirt (and ideally pockets!). I also prefer to not need a jacket if poss, so unless it's warm I generally go for short sleeves. I also quite like a big print now, whereas years ago I think there was a bit of a feeling of not wanting to upstage the bridal party. Lastly I'm more likely to wear flat sandals the whole way through now, rather than just on the dancefloor, although I do have two pairs of block heel sandals that are great for weddings - neutral/tan suede and pale gold leather.
In the spirit of using what I already have, therefore, I'd maybe try and get away with a nice full-skirted cotton broderie midi dress I wore to a cool London evening wedding last year,.or a printed coral wide-legged jumpsuit I wore to a church hall wedding last spring. I'd definitely be going for a bit unstructured, not too fussy, and probably colourful, although that's not a given.
Out of your first links, OP, I'd steer away from the more slinky evening type gowns. I like the pale blue Reformation one you linked. I like a co-ord, but the black Cos one wasn't for me.