How about an advanced option that enables us to write our own CRON schedule?
Maybe even multiple very specific CRONs (that would enable a schedule such as “last day of the month”):
55 23 30 4,6,9,11 * myjob.sh
55 23 31 1,3,5,7,8,10,12 * myjob.sh
55 23 28 2 * myjob.sh
I understand that that would require some validation, but it allows for a lot more flexibility than currently possible on the front end.