Ok,
I must admit, I might sounded a bit harsh.
The GUI on full screen of BIAB is almost ok looking from first sight.

I do believe it still needs scaling / modular approach., where user can design and save the workspace (upper tool part) which includes the items to be displayed, their custom sizes, etc. I would allow for double the size for tools at full screen to be at user disposal.

Mixer is another story...I will mention only visual part of it. User Videotrack showed an idea of auto-hiding mixer some time ago. So far, I was not able to see or come up with a better idea. Sorry, I can not find the post...

Menus have to be groomed and sorted better. Seems like they are scattered everywhere. Some roundness in menu styles and less of 90s look would help visually.

These are very doable things and out of all complex items on the lists are probably the less time consuming.