Ballet flats and similar shoes are a challenge if someone needs foot orthotics or some sort of arch support in the footwear when there is such limited room in the footwear.

There are some options.

Curated Resources:
https://onthespotarches.com/arch-support-in-ballet-flats/
https://croydonfoot.com/how-do-you-get-some-arch-support-in-ballet-flats/
https://articles.abilogic.com/457976/how-get-arch-support-ballet.html
https://podiatry-portal.com/onthespot-arches-for-instant-arch-support/