It's not completely inconceivable that Poland and Germany would have allied, during WWII Germany was allied with "Slavic" nations like Hungary, Romania and Bulgaria. Poland even partook in the dismemberment of Czechoslovakia in 1938, annexing parts of it. I think a bigger stumbling block was that Hitler viewed the parts of Poland Germany lost after WWI as rightfully German territory, something that can't be said of e.g. Romania.

>Was it a realistic scenario at one point for Poland to join the Axis powers or was Hitler’s plan always to invade Poland and subjugate it due to Poland being controlled by the Slavic people who Germany saw as beneath them?

Short answer, possibly, with some major reservations.

Germany wanted Danzig back. Historically, Prussia/Germany controlled the northern Polish coast up through the Baltic states. It lost the Danzig corridor in WW1 and had this German exclave that it wanted to connect again.

Could Poland have been a full Axis member despite being Slavic? Probably. Italy, Hungary, Bulgaria, and Romania all got to join in and none of them were Aryan.

In an alternative history, could Poland have joined the anti-commiterm pact with Germany? Sure. But that was 1936. By 1939 when Hitler wanted to expand - this is post the Anschluss of Austria and the Sudetenland - he wanted Danzig. So, perhaps Poland could have given up the Danzig corridor and appeased Hitler and avoided the need for the Molotov-Ribbentrop pact between the USSR and Germany to divvy up Poland.

But what that would end up looking like for Poland is being stuck between two very hostile neighbors. If Poland was aligning with Germany, then like other European Axis minors, there's a lot of deference being given to Germany. More than likely, they still end up being the front line of the German-Soviet conflict.

Long term it's harder to guess at, but you'd likely see Poland become a puppet of Nazi Germany, similar to Viche France.