Google Flights is admittedly wonderful, but sometimes it is necessary to "cast a wider net". Other OTAs like Expedia, Orbitz, etc. Airline specific just in case one has specials not in the OTAs, or there are airlines on the route not in Google. ITA matrix for a little more specificity than Google Flights.

In your example you've already found really significant savings and presumably have a good general idea of "reasonable" rate levels. It would be buy immediately time.

But if you hadn't found what you felt was a good deal, that is when the wider net comes into play. And there also are the "alerts". Lots of time between Aug and April. Set several of them and hope for a sale, being prepared to act immediately. If nothing shows up before rates likely to start rising (minimum 6 weeks ahead), give up and buy.

Lots right with your methodology though. Probably the quickest, simplest, wide search available. Can't argue with your results.