My perspective on designing a rack is not only focused on sound and utility but also the visual impact from an artists perspective. Understanding the utility of the modules should be foremost in your thoughts. To me the design & look is equally important.
With that in mind I will either work from left to right or place the larger modules dead center and work toward the sides. Working with modules that are the same color and size or from the same manufacturer helps me design something that hopefully draws me in visually and makes me want to create art.
Some of my designs are simply utility racks while others are fully functional synthesizers. I go through many revisions.
They usually remain private until I feel the design and balance is what I intended.
With everything available today we have the ability to generate wonderous sonic adventures and look good doing it too.
I would start by putting all your black modules in the center row. Put your large filter bank in the center & each of the oscillators on either side of it. Now that it is centered work toward the edges with the remaining black modules and blanks. This creates a balanced look to the rack. Blank panels come in all sizes. Instead of just adding a blank panel to fill a hole use them as spacers to create a uniform look. When working toward the sides try to work in the same direction on each side using modules of the same size. The doepfer wasp is to small for a center piece. I would actually buy a second one just so I could have one on each side of the oscillators. You can try anything imaginable on this site. If you keep moving things around it will come to you.
Move the doepfer sequencer to the bottom row and place the doepfer envelope generators and small mix modules on either side .
Keep the overall apperance in mind as you design it. Use the small 2 or 4 hp modules as you work toward the edges to keep the look consistant.
Save blanks for the far edges of the rack or use them on either side of the center modules to create an overall balance or visual separation.
I rarely use blanks for anything other than a balanced appearence.
Center the behringer mixer on the top row above your black filter and work to the sides with your Maths and other remaining modules.
You can try any combination you like and swap out blanks and modules until you are satisfied with the final look . I dont worry much about the utility and patching. Cables come in all sizes. I look at the design of the rack as a work of art first and foremost. The design should draw you in like a museum piece. People not familiar with modular synthesis will assume you are either a scientist or electronics technician unless you give them reason to just go WOW what is that?