Since 'formal' means black tie for men, then why not black tie for women in the year 2024. I think the white suit will look stunning too, just bling it up.
Of the two dresses that you showed, the Roland Mouret looks very nice but still a bit cocktail, if you're not tall perhaps it'll hit the leg lower though. McQueen, Westwood, Balmain, Maison Margiela, Dolce Gabbana and Victoria Beckham to name but a few also offer tailored evening wear that is suit-ably formal.
Of the two dresses that you showed, the Roland Mouret looks very nice but still a bit cocktail, if you're not tall perhaps it'll hit the leg lower though. McQueen, Westwood, Balmain, Maison Margiela, Dolce Gabbana and Victoria Beckham to name but a few also offer tailored evening wear that is suit-ably formal.