concentric twist advice

I'm working on the main engine harness for my car. I'm getting ready to do my first concentric twist and had some questions on my setup. I have 2 shielded cables as my core. They are 3 conductor M27500-22SB3T23. I figured I would use that as the core then the 20 gauge wire for the injectors as the first layer then work my way up to the 22 gauge wires in the second layer. When I twisted the core I found it has big gaps when you twist them together. Attached is a picture of my test core. Then I decided to test a few things. I found if I twist in 6 20 gauge wires in the gaps it will round out the cables pretty well. I haven't tested it with 22 gauge wires which might work better. I attached another picture with me adding the filler wires in the gaps. What I don't know is if this a bad idea for reasons I can't think of or is there a better way to handle it.

I regularly add smaller wires into made a core more round. No issues doing this, I usually use say a 14 awg to feed ignition coils or 18 awg to feed 12V somewhere and bury that in the core layer until it's spliced in a junction.

Concentrically twisting is this puzzle of where you put wires and how big. For example you could decide to leave the core like that and feed your ignition coils with x3 20 awg wire and splice them together. Or you could move those into the core and have room for more in outer layers. It takes a bit of playing around with how many wires you need and what sizes they can be. Using something like Excel or Google Sheets really helps me here as I can plan out multiple options and choose the one with the least compromises, or splices, or filler wires.

I'd say your twist is a little tight looking, try just twisting them gently and see where they 'like to lay'. Also trying twisting them both directions, the conductors inside the shielded cable is twisted and they can sit more flat in a Left Hand lay in my experience.