Hi and welcome to Trac!
Because this involves the block editor interface, these reports belong on GitHub, if they are not already there.
The Gallery block has left and right arrows to reorganize, but of course that is not drag-and-drop.
For now, I recommend experimenting with creating galleries in a "Classic" block. Use the "Add Media" button and then select "Create gallery" on the side. That interface has drag-and-drop functionality, and it should not make image(s) in the last row larger/wider than the others (unless your theme creates that layout).