If you don't want apps to be able to see each other or communicate, put them in separate profiles; that'll do it.
It is planned to eventually have a feature to restrict app communication for apps residing in the same profile, but it needs to be properly investigated and implemented. GrapheneOS won't ship a half-measure solution.
You can see the issue for it on GitHub here:
You can also get a glimpse at what that may look like in practice here:
cactus457 Moreover, I wondered if it is possible to install apps (via the sandboxed google play store) in a different user profile than the store itself to prevent communication between apps.
If you install Sandboxed Google Play in your Owner profile, you can then use the "install available apps" feature to move them over to other profiles without having to install them there separately.
Furthermore, if you update an app in one profile, it's updated in all.