I don't like the Jolie Moi dress and your own outfit of heavy, polished cotton is a much better material than it and sounds nicer.
Re getting a new top for the skirt, if the skirt and top are separates but are the same brand and same material intended to be worn together I'd leave them as they are.
You don't need a new dress for something like this if you don't go to that sort of thing regularly. I've got a heavy, silk jacquard green and navy embroidered dress I've worn to several, including annual repeats of the same event. I've got loads of day dresses but I've no qualms about repeating formal dresses.
I can't recall what anyone else was wearing at last year's ceremony of the one I'm going to next month. If someone remembers mine, then the dress did a good job of being noticed. I've worn the green and navy dress to at least three of them. I can guarantee that absent weight gain, all the men have been wearing the same black tie suit, every year.
I think if you want to spend or treat yourself you'll get better use of your money either as you suggest or something you'll wear more often.