In years past I tended to book flights as soon as I knew our itinerary, but I'm not sure that was always the best price.
I tend to do the opposite - wait til I find what appears to be a reasonable price for a flight, book the flight, and then book all the accommodations and seal the itinerary. I like to do this as early as possible, and here I agree with NoSpin - about 9-10 months before the trip. If I see that the accommodations might be hard to get if I wait, then I'll book them before I've booked flights, and cancel if things change. I will cancel many months ahead (at least six months) in order to be "fair" to the accommodation (always small places). Most accommodations have reasonable cancellation policies, unlike airlines.
So this way of planning allows me flexibility in choosing flight dates, which in turn enables me to choose flights based on price.
What's a "reasonable" price? I, too, rely on Google Flights' recommendation - plus I track prices myself the year before. For example, if I am planning for October 2023 to Italy, I will have started to look at prices for October 2022 in the summer of 2022. Also, because prices for October 2023 will not be available yet, I track prices for March 2023, which I tend to look at as a "parallel" month to October - March is just before a major tourist season, and October is right after. March 2023 is enough in the distance from summer 2022, that it can serve as a comparison to October 2023. I may be wrong about that, but so far the comparison seems reasonable.
Yes, a bit of homework - but these days you have the relevant sites sending you tracking info, so it's not as much work as it sounds.
Bottom line - around 9-10 months before the flight, I have a fairly good idea what a reasonable price could be. It won't necessarily be the lowest - for that I'd have to play a waiting game that I don't like, as there are always sudden bargains that appear at some point - but it will certainly be a price that will be on the typically lower side. Having the whole trip booked around 9 months in advance (both flights and accommodations) gives me quiet - which I suppose is also worth something, at least for me.
No use in buyer's remorse here - global fluctuations have become so unpredictable, that I consider myself lucky if I will actually be able to make a trip next autumn, even at a "reasonable" price. You have to count your blessings.