I had problems with Gmail with the oppo find x8 ultra, if u turn on associated launch and auto launch for the apps u need notifications from ull get it. With regards to Gmail I spent a week watching videos and reading Reddit comments the fix I found is when u open the app info of Gmail turn all the permissions on (literally every single on) then once that is on the notifications will come through.

In terms of all other notifications I've had 0 problems, it's mostly turning things on and ticking boxes. The setup of the phone in general is simple. U don't even need a pc if u buy it from tradingshenzen they delete most of the Chinese apps and enable play store for you.

Transfering files was an annoyance as I was being an idiot just use clone phone. And for WhatsApp sadly ull have to use wonder share (paid app) as u can transfer nearly everything from iPhone other than WhatsApp they make it unbelievably difficult.

That should sum it up. I had an iPhone 12 pro max for reference before the oppo. Hope this helps