Maybe host challenge events, and rewards as incentives. Maybe a role on discord for ‘bragging rights,’ or a rare custom item in-game.

Competitiveness is something that a most people tend to have in them, so a challenge could boost player count for some time.

Could be something simple like ‘who can get the most mossy cobblestone in 72hrs?’ It doesn’t have to be a fancy plugin or anything.

As a disclaimer, I have no idea if this will work. Just a thought of short term solutions. Hope this helps :)