Would be nice with a smaller more minimalistic window that takes up less space on screen.

Perhaps something like this? (Pardon my paint skills)

https://i.imgur.com/lObMIJr.png

The problem is it doesn't really fit well there. It'd be too wide. Perhaps aligning the entity icon on the left, then number and quality icons stacked to the right? Something closer to the original GUI

You mean too wide without scaling down the icons themselves? Yeah I see, though I would be ok with a slight downscaling if possible, so that it fits. Or omit the entity icon for above normal so it takes up less space?

https://i.imgur.com/lchv02V.png

Or i guess if that's not gonna look good, just making the original window slightly slimmer overall would also be appreciated. There's a lot of padding inside the GUI that could be removed? :)

Too wide, yeah. It would need to be the width of the GUI below, no wider. It's hard to scale down the item icons but that might work.

I can make the main window slightly smaller, but not by much.

Right ok. what about potentially just a transparent GUI, so it's just the 5 icons shown on screen? Then I could position them above my toolbar and it would look more minimalistic still.

https://i.imgur.com/pdmHbtp.png

Then, also add an option to auto hide when not mousing over an entity, which would compliment the option you already have for mousing over to open the window.