I assumed to be avoiding the penalty of task switching, I'm not well read on the topic but I assume it's more pronounced trying to do X different tasks per day, than segregating one focus per day with a sleep in between them.
That being said, your comparison of time/effort ratio is very apt. Frequently I've found myself having to temporarily halt the cycle in order to focus on cranking out one project that needed much more attention before some date. The rates of each project progression are not equal, and a simple split does not address this at all.
The fear of focusing on one project at a time instead of juggling them is that each project's completion is probably on the time scale of many months- so to neglect all other projects in favor of one- for chunks of a year at a time, seems like it would be very stress inducing.
Maybe it's a good case to consider some compromise between the extremes, something less frequent than daily switching, perhaps weekly, biweekly, or more is a more appropriate time scale to approach this with.
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
123456789012345678
AAAAABBBBBBBCCCCCC
ABCABCABCABCABCBCC
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.