I also have the same package and issues as you described since October last year. I've had a case open for months now and no one will give me a straight answer.
I agree it is down to utilisation on the network, the traffic or routing is just not satisfactory to go un-noticed. All games with a packet or latency indicator behave the same. Overwatch, Warzone, etc, I'm on my 2nd router and 4th Engineer call out now. Download and uploads speeds fluctuate massively during peak times. I said this to the engineer and they just fobbed it off like I was talking rubbish.
Good luck if they fix it for you, be good to know exactly what the reason is?