Another powerful, flexible and customisable admin panel:

https://docs.sonata-project.org/en/master/

Another REST API:

https://github.com/FriendsOfSymfony/FOSRestBundle

Don't make the (big) mistake of choosing the two options you mentioned only because Symfony started promoting them recently by adding the bundles to their documentation. It's a promotion.

There are other solutions with millions of implementations that are extremely powerful if you invest the time to investigate them.