Charge by creation or by month? (Patreon)

Based on the high turnover rate of patrons since I started back in September of 2017, I'm wondering what I can do to improve patron retention and provide more value for pledges.  I'm considering moving to a monthly charge instead of charging per finished image.  This likely means that patrons will be charged LESS per month.  Of course the amount of work I produce each month varies, so some months patrons may receive less work for their pledges.  I would appreciate your feedback, whether you are a current patron, former, or considering.  Feel free to comment as well if you have other ideas.  Thanks!