Why is this the minimum width I'm allowed to size the right side of the app to?

If I try to reduce it's size, it snaps to being collapsed.

What's the use for all of that wasted space that takes away screen real estate from my drawing?

I would close the extra tabs.

It's also unclear to my why your GUI is so large... large text, large icons... on a 2400x1600 display. 🤔

Here is 1920 x 1080:

Not everyone has great eyesight or big monitors.

You could also try moving the panels out into separate dialogs, by dragging the tabs or using the dropdown menu at the right. Then you're free to position and size them independently. This is how I prefer to use Inkscape on Linux, since the window managers usually do a good job of working with floating dialogs, and it gives me a lot more flexibility. On other operating systems it doesn't always work so well. I know some users just prefer a permanently docked UI anyway, so you might have already dismissed this approach - but it's worth mentioning in case you weren't aware of it.