I've been meaning to try android and graphene OS. Bear with me because this might be a silly question: is it possible to fool around with it and get comfortable before moving my phone number over to it? (I.e. if it's not connected to a phone number right away, can I still experiment with apps?) 

 Yes, however you would not be able to test calling, SMS, navigation, etc. I did this before I switched. 

 It is possible to get a prepaid SIM for pretty cheap and test everything. 

 I was an iOS user since 2008 and switched to GrapheneOS this year. Love it, no regrets.