I personally do not like the 30/31 idea. I always use 6 rows and fill up the blanks with ghost numbers - dates in the previous / next month.
I have templates developed over several years which reflow all the dates in linked text boxes. I only need to change January 1st, add Feb 29th for a leap year and move Easter and it’s done. Unseen ghost dates are in boxes off the page so the reflow works properly.
There is a second format to consider, where the days of the week move and the dates are static, in vertical columns. In this one the only spare space is the one, two or three blank lines at the end of the last column.