I plan to arrive 1hr 45mins before my flight from either T2 or T5, with Fast Track in both and 9 times out of 10 that gives me about 45 minutes in the lounge and still time to be in Group 1 for boarding. I find that this is the ideal balance between no stress and not wasting copious amounts of time.

The 1 time in 10 is when transport is disrupted, but there's enough slack in that to allow me to switch modes - eg get to Paddington for the Elizabeth Line instead of taking the Tube or, once when all trains were suspended, get an Uber.