Rin

One change at a time, new muslin each time. Slower, but if you change three things at once and it gets worse you have learned nothing.

Two wraps of tape being a genuine fix rather than a workaround is not what I expected.

It went in about four months ago, so that is overdue by most people's numbers.

I definitely guide from behind. Possibly with more enthusiasm than guiding requires.

Long basting stitches by hand, or a machine basting stitch?

I have unwrapped rotors with a genuinely oily film on them. Always clean them.

Worth checking the jockey wheels while you are in there. Sloppy upper jockey adds noise and vague shifting, though it will not cause the skip you describe on its own.

Have somebody else ride it while you walk alongside. Creaks sound like they come from the BB because the whole frame is a sounding board and your ears are three feet above it. From outside you can often hear it is the saddle.

At 120km a week in all weather the cheapest thing you can do is wax it, or at minimum clean and lube properly. Wet chains at 6000km look like dry chains at 3000.