Personally I would prefer the train option here. For September, you're buying well in advance so you could set an alert on the Avanti website to book when seats are released - currently selling until early September. If Standard Premier or First is not too much more, that's a very comfortable way to travel. Standard is also absolutely fine.

The downside of the train is the connection from Euston to Heathrow which isn't ideal, but not terrible. You can either take the Northern Line down to Tottenham Court Road for the Elizabeth Line, or walk to Euston Square and then HEX to Paddington.

If you could get MAN-LHR-LAX on one ticket for the same price, I'd maybe take that in preference, but (guessing you're tied to Star) the two shorter long-hauls via YYZ isn't as good in my opinion.