Congratulations :)
BTW, if on any point of your journey you decide you want to go back to stock, it's possible and easy (but your phone will be wiped so you'll need to make a backup of your data).
Now to the point -- did you see https://grapheneos.org/usage#sandboxed-google-play ?
Re: mirroring: it pretty much means your phone doesn't talk to the Google servers to install these APKs, it uses GrapheneOS-provided copies - without any modifications.
And concerning "using GOS for safety", it might be useful to remember it's not magic, and a lot depends on your behaviour/pattern of use.
See more about what GoS offers: https://grapheneos.org/features#grapheneos , that might give you some ideas.