Going against Poland first would probably have had for consequence that the system of alliance between France, Britain, Poland and Czechoslovakia would have been more effective.
Matteo, so your thinking is that if over the course of a war with Poland, Germany shows its hand by occupying huge swaths of ethnic Polish territory, that British, French-Czechoslovakian cooperation will tighten up, and London and Paris won't try to sidestep confrontation, so the Germans won't get Sudetenland and inner Bohemia for free?
The optimistic scenario is that having initially tolerated German aggression to revise the border, London and Paris decide a firm containment policy is needed once Hitler conquers Poland. London and Paris draw the line at Czechoslovakia and the Balkans. When Germany attacks the Czechs, the west declares war. Even if it's a phony war, getting through Czechoslovakia is more of a slog than Poland was, and seeing the Germans tied down in this way, perhaps the Soviets and Romanians even intervene to aid the Czechs, initially by invading German-occupied Galicia.
Even if the west decides to draw the line at Czechoslovakia, a more pessimistic outcome is that with German troops ringed around pretty much the entirety of Czechoslovakia's southwestern, northwestern and northeastern borders after Germany beats Poland, the Czechs conclude that there is no hope for military resistance as all defenses can be severely outflanked and the western powers can't really do anything about it. And the Soviets decide they don't want to risk involvement and be stuck with the hard work,
So, the Czechs give in without a fight despite western & conditional Soviet pledges.
Then the west is stuck trying to draw the line in Romania, which may or may not work.