Using graphene as daily driver for years, you can still install all apps you previously used, no need to replace everything.
Discussion
You mean as a first step, or just as an end goal?
Of course its great if you can replace something with an open source solution, but for me personally its ok to have some "evil" apps like google maps, Snapchat etc.
They work fine and you still have the increased security of grapheneos (sandboxed apps, hardened malloc, overall security focus).
You can also disallow background execution for apps so they immediately get killed once closed and can't call home all the time.