Reccurent task planning but not on an weekly baseline

No, no planning regular.
Only weekly regular exists.
you must plas a main task wuth sub-task for each period.

But this would be more complex to define an manage.
I know how to manager 0.5 day every friday : easy.
But manager 0.5 day the 10th of every month will be a mess : what should I do if it is a sunday ?
The only "easy way to define regular for months is "every 1st monday of the month" or "every last monday of the month" or "every first open day of the month" ...
It is not easy to define on a calendar basis, so much harder to define that weekly regular.

Moreover, montly frequency means you just have to define 12 times a year, so either you define an main activity with 12 subactivities, or just use a manually planned activity with 12 selections of periods...

So I see very small interest to manage so complex feature