Google Flights search is irredeemably broken for long international business-class flights, and I've tried a few times over the years to get someone there to give a shit, and they will not.

Example: You search for a flight from YWG to BKK in business, and it wants to route you YWG-YVR-KIX-BKK, which is basically fine. The flight time from YWG-YVR is under 3 hours, so it'll show you fares where that leg is in Economy, but YVR-KIX is in business, which is also, fundamentally, fine, although it would be nice to filter that. A short hop being Economy to get you on to the long leg in Business is usually acceptable. HOWEVER, it will ALSO show you fares where KIX-BKK (a 6 hour flight) are in Economy, and it won't allow you to filter this, so this messes up any ability to sensibly filter flights by price. If I am searching for a flight in Business, please allow me to filter out ones where *9 hours* of that flight is actually in Economy.</rant>

I imagine you could quickly build your own browser extension that parses all visible segments and hides cards where any individual leg exceeds your economy/business threshold. I successfully vibe coded a solution to a different Google flights problem in about 10 minutes.