Hydrolysis of PETG causes brittleness as the polymer breaks down when processed with significant moisture content.

As does excess heat.

Since most people don't have a major problem printing filament that's wet by my standards (and the manufacturer's) I would wager that the heat decrease will improve things on your larger prints.

Also be aware that thermal degradation is a function of time as well. If your layers don't have enough time to cool, heat can possibly build up to just below melt temp which can cause it too, but you would have to have some very very hot layers to do that.

With that said, please do report back with results. Too often people search and find a thread with their issue but it's never resolved as the OP comes back and says "got it, thanks" and never says what or why.