I turn 60 next year. I don't think it's a matter of age so much as a matter of individual taste, & also one of personal style (& whether you care about looking dated or not).
The OP sounds quite diffident about her own style - understandable when she's lost so much weight! But she also doesn't found very sure what colours & shapes suit her.
Both her husband & daughter admitted to hating the dresses she tried on. They're generic overpriced, dated-looking, MoB dresses that don't fit the OP properly, do nothing for her newly discovered figure, & could have been purchased in 1986 not 2026.
From the style of the Susannah dress that the OP posted, she isn't wanting anything cutting-edge or out there in any way. But she can still wear a lovely, well-cut dress that is classic but also looks modern, not dated.
Facebook Marketplace & Vinted are FULL of these MoB dresses (& matching jackets, & hats/fascinators ...). The wearers are desperately trying to sell off something that they'll never wear again, which cost several hundred pounds or more, for a pittance. And they languish there for ever ...
The OP could easily find something lovely that, minus the MoB accessories, could be worn again for dinners out, birthdays etc. Much better value for money too, if she's going to spend £600 on a dress, to be able to wear it lots afterwards!!!