Your special category condition is that forum posts are manifestly made public by the act of posting them.
Also you’re not doing much processing about their opinions, right? Just letting people talk to each other.
so: write that down!
[DRAFT] We are aware that this is a forum focused on political discussion, and that political opinions are “special category” data under the EU GDPR and other regulations. As this is a publically accessible forum, you should be aware that observers may be able to identify your political opinions based on your posts. Processing of this data is allowed under Article 9(2) condition (e) - the data is made public by you posting it. [DRAFT]