I am new to Lexus and this forum, but in 30+ years of car ownership the "it's normal, they all do that" is typical of the response from the UK motor trade when they can't diagnose and successfully resolve a problem. My 2018 RX450h with 12,000 miles does not do this and no one else has piped up either to say it's normal. As to the 75,000 miles statement that is just rubbish, a car of this age and mileage should either function correctly or has a fault.
To me it looks like you may be running on only 5-cylinders when this occurs, this would explain the excessive vibration, but I would expect this to be easily diagnosed and could be due to a spark plug, coil pack, injector, wiring etc. I would ask them if they are 100% sure this is not the problem and also get the service manager to demonstrate this on another vehicle as he has stated.