You could maybe later on consider subscription based on usage and max number of req/month ...
That was one of my idea to keep SteemSQL affordable for some low usage people. But it's quite more complex to implement. This is something I might do in the future.
Steemsql looks like great work and should be rewarded
If you are looking for potential "mini" subscription solution
You can use Logon Triggers to ex. limit the number of 'mini' user logons to 1 a day or week (this should be enough for people who just want to run single report )
Logon Triggers doc -https://docs.microsoft.com/en-us/sql/relational-databases/triggers/logon-triggers