Thanks @Anonyme for the welcome to the community, the idea to downgrade to stock Android 8 and the provided links.
Sadly it probably will not work as @harvey186 and @archje commented 
Thank you two for pointing that out 
I also wondered about Archje’s comment and /e/ in general: If I would downgrade to Oreo I would also miss the important security upgrades of 2019 till now, correct? Does that not pose a security threat or are you doing anything that helps mitigate the associated risks?
If I understand it correctly, the security updates are also the reason why the developers of GrapheneOS only support devices that are still maintained by the manufacturer.
Should I as a user worry about this?