I would go with a smartphone.
Preferably one that support CalyxOS, GrapheneOS, /e/OS, SimpleOS or iodéOS. This way you will have access to secure messaging through Signal (and other) you will be able to install your work app in a Work Profile (you can use Shelter if it’s not supported by your ROM). This will be more secure and privacy firendly than a dumb phone.
Installing a minimal launcher such as OLauncher (or the fork OLauncher clutter free) and having only the apps you need on it. This way you will make your smartphone dumber and more productivity driven.