Hello ethan_nx and mOgliE,
I think you guys have been doing a great job by running a long campaign. But I believe that you are just encouraging members to make unworthy posts and paying them. I believe for a user who is making 20 posts a week earning 5 merits from 300 posts is not a difficult and that earning atleast 5 merits in the last 120 days is a reasonable measure to identify a user as not being a shitposter. Do check your participants -
hahay - 0 earned merits in 120 days. He hasn't earned a single merit since the art contest. Even with the art contest which earned him 19 merits, he had earned only 33 merits in the last 2 years.
onrise - 0 earned merits in 120 days. Haven't earned a single merit since September 2019. He had earned only 32 merits in in the last 2 years.
FlightyPouch - 1 earned merits in 120 days. Earned a total of 38 merits in the last 2 years.
maydna - 3 earned merits in 120 days. Earned a total of 9 merits in the last 2 years.
Janation - 4 earned merits in 120 days. Earned a total of 16 merits in the last 2 years.
I think you guys should seriously look into the participants and promote your site wisely. But this is just my thoughts. You can run the campaign however you like because its your money.

Cheers,
Luci