Most people on this forum do not listen to the podcast.
Ah, shit... I got tunnel vision from my own experience where your podcast came first before I used the site.
Probably good to start with "carrot" informative banners. Maybe below the sponsored post banners to start with. "Get shiny badge and banner and your post could be here! Starting at $20/month". You should find out where user's eyes go the most and try advertising there. Maybe above highlighted thread posts or high-engagement posts below the ratings. "Unlock these ratings today!"
Now for the stick. The Wikipedia method might be your best example. Constant stimulation of a "crisis" and empathy baiting to make people feel like a bad thing is happening to them to get anxious (and their money can make the anxiety go away) does work on people. Normal charities try to use "positive empathy" stuff summarized in what is probably an AI Sloppah summary:
https://www.spotfund.com/blog/ncSA7w-emotional-engagement-in-fundraising
BUT, online communities are more susceptible to second-hand anxiety. You will see that is what Wikipedia started doing to GREAT effect.
"We are not for sale and can not allow it to happen!": The site is at risk of being conquered by Elon Musk, your enemy.
"Only <1% support us.": Nobody is helping, you are alone.
"Democracy and freedom depends on us.": You will be thrown in fascism jail if we don't raise money.
"We are only at 17% of what we NEED.": We are going to fail because you didn't help.
AND they blast this at them every time they load the page. Anxiety generator. When users donate they excuse themselves from the banner AND the anxiety. Essentially, Wikipedia gave them the disease and then offered the cure.
It is disgusting because Wikimedia is a scam investment fund. In the forum's case, it really is at risk, so it is fully ethical for you to be honest to the users.