recommendations

https://www.mangago.zone/read-manga/miss_not_so_sidekick/
is an enjoyable read, though I don't really remember any particularly amazing dresses. But to be fair, my memory is really bad.

https://www.mangago.zone/read-manga/kill_the_villainess/
truth be told, I have yet to finish it myself, but I do believe it's one of those manwhas with really pretty panels and dresses. Still, my memory isn't that great. However, both of them are completed romances in a historical setting, so they at least fulfill that aspect of your request. Hope you enjoy them! :)