You need to define "dumb phones" and "privacy" first in order to come to good answers.
Dumb phones: Are they old phones only capable of SMS, calls and playing Snake? Or are they modern Android phones with a display and keyboard but limited in features? The latter ones will probably be much worse compared to your primary (or a secondary) GOS device, and even the former lack sandboxing and security features compared to GOS.
Privacy: What kind of privacy do you want to achieve? What data do you want to protect against whom? Using a SIM card at all will expose you to your carrier, but on GOS it's more easily controlled (e.g. Air Plane mode means cutting off any data transfer). If you don't use your SIM number (e.g. not sharing it with online accounts for verification or Banks for 2FA) maybe you don't need a SIM card at all. If you need mobile data, a mobile router with a prepaid data SIM card may be more feasible, because you won't share your GOS device identifiers with any carrier. But maybe all you want is to just minimise your exposure without any inconvenience, and using mysudo or alternatives like jmp.chat while sharing your cellular number only when necessary is the best approach.
So in short: Make a threat model and don't trust any best practice approaches since everyone has different needs and unique definitions of privacy.
Good starting points:
https://www.privacyguides.org/en/basics/threat-modeling/
https://opsec101.org/
https://grapheneos.org/faq