Thanks for the kind words Martin.
When I work on a retainer I choose to use weekly time increments because I find that’s the best balance for estimating time while remaining flexible. Monthly may work fine for you. It depends entirely on the size of the projects you are doing, and your internal processes for how you schedule your time between your different projects. One advantage of doing it weeky is that it’s easy to add a remove a week from the schedule it the project needs a bit more time.