If the point of getting a forum site running on a Raspberry Pi is just for novelty & challenge, ignore this. But I think the most practical solution here is to

If the point of getting a forum site running on a Raspberry Pi is just for novelty & challenge, ignore this. But I think the most practical solution here is to forget the Raspberry Pi altogether or to set up the majority of processing and storage on a separate machine and have the Raspberry Pi server talk to it.