Do you rotate to keep your motivation alive, or because of other reasons (e.g. let a topic simmer for a few days, so you get inspired / or reducing risks by gaining more information or knowledge over time)?

Because multitasking is the worst possible.

Let's say you want to finish A which lasts 5 units of time/effort, B which lasts 7, and C which is 6.

Compare Sequential vs Multitasking

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Sequential: A done at 5, B done at 5+6=11, C done at 5+6+7=18

Multitasking: A done at 13, B done at 16, C done at 18.

And, that doesn't count the task switching costs.

So choosing multitasking only makes sense when you gain something by waiting, or have external constraints. In the context of self-employment, keeping your clients happy and calm with (the performance of) steady progress (even if it costs more than faster completion, if done in a more efficient way) may make it worth if you have to take multiple concurrent jobs at once, e.g. if you need to involve them in decision making that stall your progress.