> Each user would than pay the fee (maybe 1 SBD) for each used account per month.

If you move to this model, you would probably gain more participation if you accept Steem instead of only SBD. I can't remember the last time I was paid in SBD (not that I post regularly) and would be easier to transfer the Steem that is already in the wallet than needing to go exchange it for SBD. 

I don't know the costs associated with running your program (which isn't even examining the time you are putting in for coding), but you might consider implementing a way for users to donate for others who either don't post a lot, or they do and receive little support. I see a lot of accounts out here that are lucky to make 1 SBD in a month and hate the thought of them losing access to your tool. 

Thank you for the service you provide the community, and hopeful we can all come together in some way that rewards you for this service.