i would really like to be able to see my entire shulker inventory without the compact mode.

There's a combination of using swapModes that if you set to true will do what you want. I don't have modmenu installed on my setup at the moment so I can't give you the exact settings needed but you can definitely get the full preview mode to always show by default.

If you do the swapModes suggestion I made and always have the preview set to true, you'll always have the full preview available no matter what. I had made a mod that did this exact same thing but found this even better implementation. I hope this helps you and I can't wait for this mod to update to 1.20.

As @zenenwjaimes said, you can enable swapModes to always show the full preview by default. If you want to never see the compact mode even when pressing the full preview key, you can unbind it (by pressing escape) in the Controls category of the configuration menu.