Love my OnePlus 6, it hasn't aged one bit since I bought it. Glad to see development happening in an "old" but capable device.
But the battery starts to become weaker - and I have some trouble with the charger (it's on the device, not the cable). Too bad it's always so hard to exchange battery in these devices.
If you're a tinkerer, I'd seriously recommend using something like ACC (Advanced Charging Controller). It'll automatically cut your charge off at ~80% and slow the charging if it's heating up too much. Should greatly extend the service life of your battery. You can pretty much just install with Magisk and forget it. Perhaps it's too late for your current phone/battery but maybe on the next one.
Pair with something like AccuBattery for visibility into battery capacity over time.