A
Welcome.
Uninstall Oreo? Not without effectively resetting the device.
For the notification badges, have a peek at this thread - https://forums.androidcentral.com/s...app-badges-not-showing-numbers-galaxy-s8.html
As for missed calls, what alert tone are you speaking of? I'm not sure I've experienced a distinct tone when I missed a call, except for the ringtone ending...
My phone also goes to sleep while I'm on a call, either because the set amount of time passed or because I moved it near nd it's not near my facemy face. Are you expecting the display to remain on for the entire call?
Also there's an issue with the Gmail app where if you dismiss an email from the shade, you loose the notification in the app
Not really an "issue" as it's the way it's designed by Google. This is how my Pixel 2 XL works with the Pixel Launcher. Keep in mind - Oreo is the first version of Android where this capability was built into the OS and not added by a manufacturer.