Don’t focus too much on exact diameter. I like 0.3mm, but smaller or larger will do just fine. Try a piece of lamp, ethernet or speaker chord and see how it works.
Redesign a version with standard Arduino interface should definitely be possible. Quickest way to test the boards is probably to solder two pin headers or sockets instead of the pro micro. Then you can use jumper cable to connect it temporarily to any board of choice.