I was shaping such system myself and after some time I understood that global kanban boards are enough, but what is more important is how do I focus (and return to the focused state) during the work.

To focus and return to the focus better, I'm using a mix of:

- project microtasking with kanban board (automatically handled using scripts/app) and TODO/FIXME comments

- lazygit utility

- global todo kanban board (think Trello, GitHub Projects etc.)

I was looking for a system that will maximize my productivity during few hours per day when I work for myself. I discussed this system before on HN: https://news.ycombinator.com/item?id=20780939