FWIW on most long hauls, my problem tends to be feeling cold rather than hot. I have settled on a "layers" approach. I start with a lightweight shirt and long pants, also socks. Then I have a sweater or a sweatshirt in my carry on. Finally the airline's blanket. That takes care of most situations. I also carry 1-2 changes of clothes with me, but that is not because of cabin temperature, rather what if my luggage got misplaced and arrived late.

I avoid shorts out of the superstition that I am less likely to be ill-treated if I look properly dressed. Similarly, due to security checks, I switched to loafers from proper shoes and to a very light weight belt.