An actress doesn't have to be born in 1991 or earlier in a film that may not come out until 2023 or 2024 to be motherly. Peyton List has lots of experience playing a motherly/big sister type for Marvel's parent company.
She'd be a much more natural box office draw for younger audiences than the Zendaya Disney pushed into Spider-Man.
List would be younger than Spider-Man.... That's wrong for Sue on so many levels. Sue and Reed are the Mom and Dad of Marvel, if there is a problem heroes usually go to them. They are the ones they ask for advice for, the ones that they depend on. Making them that young removes that completely, they no longer have the life experience. They'll no longer feel like the first family and more like a bunch of college friends.
Those ages should be for Peter, Johnny, and the X-Men. Not Sue.
Reed is a white dude - The best actor for the job would by the words actual definition, embody all of his traits. Including his physical traits. The "best actor" race argument never made much sense to me because it's incorrect by definition.
It's an adaptation. Changes can and will happen, Reed's race is not important to the character at all and can be changed with no impact to him whatsoever.
Here are the things that are important for a good Reed:
Believably Super intelligent
Leader-like qualities
Has a passion for science
Can get obsessed with his scientific endeavors
Can come across as cold at times but that is just a side-effect of his brilliance
Adores his family
Feels guilty for what happened to them
Everything he does is to make the world a better place (even when some things aren't well thought out or can backfire)
May be on the autistic spectrum
None of these have anything to do with race. The only thing physically that Reed needs is to be middle aged and have dark hair with graying temples. Something that can occur in any race.