I download the apk file from Whatsapp website to install in GrapheneOS, and I've noticed that notifications for messages are very delayed from 15 minutes to no notifications at all until I turn on phone screen and then all of the messages come through. Is there other Whatsapp users with similar issues with functionality?
Signal functions good but there have been times of small delays with messages coming through to get a notification.
Have you installed google services on your phone? they are available through the grapheneOS official "App Store" app. This should be installed before whatsapp is installed (at least that is the instruction for general apps depending on google services).
Perhaps you have done so already, but just a general advice: when using google services and invasive apps like WhatsApp, it can be a good idea to install in their dedicated profile and allow the notifications to pipe through to your main profile instead of installing both in your main profile. If you need help configuring it, let me know.
I know Services is available in a sandbox, I didn't think of that, presumably because I view it either with disdan or treat is not an option. There's also MicroG which I've never been interested in trying.