How does your customised productivity system work if you use any at all?

No system. "Productivity" doesn't describe something I optimize for. We use "productivity" as a shorthand for a metric when we don't quite know what we want or need to measure.

I focus on getting things done, correctly and on time. A lot of the time that means clearly defining what I or someone else (a boss, a customer, my wife) says I need to do, so I know what "done" means and can measure my progress. And it means deciding what not to do. After experimenting with various so-called productivity systems and techniques I decided those too easily become distractions, an example of Goodhart's Law, a neatly organized task list or project plan standing as the only output. I have worked with quite a few people who talked about their productivity systems, but mostly I saw them endlessly refining those systems, not producing anything other than obsessive-compulsive examples of bullet journals or kanban boards.

I rely mainly on memory, along with a (nearly empty) calendar and some small lists and notes I keep on Post-Its.