I made the switch when I got a new phone. So I kept both the old phone with android and the new phone with GrapheneOS. There was a transition period when I would bring both phones with me, just in case. Now my old phone is my “whatsapp” phone which I keep at home and turn on rarely. During the transition period I used my old phone number whenever I needed to provide my phone to use a service, but eventually I transitioned that to a VoIP. But, even then, many services will reject VoIP phone numbers, so I still make use of the old one.
I had to request a special scanner from my bank because the banking apps do not work with GrapheneOS. And I had to make sure that nothing important goes into my gmail anymore because google would request that I used my old phone 2FA in the most inconvenient moments, and also I don’t want to access google from my GrapheneOS phone.
I think that there are many annoyances that can and probably will happen if you try to jump right into GrapheneOS after having previously relied in the google/meta ecosystem. If you attempt to switch too quickly you might inadvertently lose access to your bank, and you might become suddenly unable to communicate with family and friends. My government’s online identification system requires that I use their app, which runs on google services, so I still have to use my old phone for that. And I have encountered situations in which the only reasonably convenient way to proceed is to download an app. For example, recently I registered for a gym that would then require me to use their google-store app so that I could identify myself when purchasing a physical card.