I use Google Play store and Obtanium to install apps and keep them updated.
I also installed Google Play for my Banking apps as I dont want to compromise on security with my banking.
I have a couple Fossify Apps installed with Obtanium.
Its your decission if you use a new Google Account or keep using your old one with your Graphene Phone, you have to check with your threat model and decided for yourself.
Others might have more detailed answers, I just wrote how I install apps on my phone.
Regardless how you install apps, Graphene is a huge Security and privacy upgrade compared to stock Android :-)