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Goldtop57 will those apps also be sandboxed like Playstore itself?
Apps are always sandboxed in Android (with the addition of GPlay itself in Graphene).
If Gmail is an issue, which Email app would you recommend?
Issue in what way? Privacy? You can use recommended private email providers. Email clients such as K9-Mail / Thunderbird and Fairemail can be used for the providers they support.
Is there any real need for Playstore? Since I didn't log in to Google yet, I installed the apps so far from Aurora Store, which worked just fine.
Play Store is more secure than Aurora. Aurora is typically only really useful for devices that are completely free of and Google services. If you have Gservices on your device, its recommended to just use Google Play.
Did I get it right that each messenger needs to be set to unrestricted battery usage?
I think you do if you aren't using GPlay for Push, otherwise not at all. See official documentation below.
https://grapheneos.org/usage#sandboxed-google-play-installation
You should give a battery optimization exception to Google Play services for features like push notifications to work properly in the background. It isn't needed for the other 2 apps.