I'm sure this isn't what you want to hear but that's just OxygenOS... well... being OxygenOS.
My OnePlus 7 Pro isn't much better. The difference being, when I unlock the phone, notifications decide it's go time.
What I'm suspecting is that the phone eventually decides to go into some sort of deep sleep and kills everything it deems unimportant, which is basically everything. When it wakes up for whatever reason (e.g. decides to sync something or wakes up from user input), it pulls notifications from a queue.
OnePlus is rated the worst offender on https://dontkillmyapp.com/. I recommend you read into the OnePlus page and see what you haven't disabled or enabled, but quickly glancing over it, you seem to have done most of the things the website recommends. They also have some sort of benchmark app if you want to give it a try. If none of those suggestions work, maybe going the custom ROM route might be your only saving grace, especially with Android 11, Google plans to allow users to be notified, on a system level, that an app has been killed, but I highly doubt custom ROMs would be that strict, where you would need this feature.
By the way, it looks you haven't upgraded to the OxygenOS 10 update, you could give that a try, provided that you're fine with having to factory reset the device after the upgrade, or else the phone will not run as well as it potentially could.