I also really like elementary planner, but its sync feature doesn’t work perfectly with Todoist … which is more or less required for me because I need my todo list on my Phone. It is going through a major revision right now so I’m hopeful that it gets even better.
Both support recurring monthly reminders. If I were you, I would try elementary’s planner first. It was designed for elementary distro desktop, but it works just fine in gnome
Btw, I decided for Todoist. The recurring reminder is on the premium plan, but I am using a good enough process: I set the recurring tasks to have a deadline of, e.g. the 5th of next month. Then, when I get there, I will set the deadline for the next month and so on. And I grouped these tasks in a group named “Recurring”.