Both is already available under the repeat options. Every xth month is to be found under Every x days (there you can select between days, weeks, months, years); this is a little bit hidden. Every xth day of a month is also there.
Oops. Sorry, had not read accurately. No, you cannot do that, only the one or the other.
Need option to delete a date in a repeating task. For example a scheduled task/date will not occur but will still happen on other scheduled dates.
Thank you for your suggestion we'll have this reviewed and discussed further. thank you for sharing your feedback with us.