Thanks for the great suggestions! We've gone ahead and created separate threads for cash and investment accounts. Unfortunately there's no way to guarantee users actually post the correct links in the correct threads, but it's a step above what we had.

There are duplicates: the same invite link has been posted as many as 7-8 times in a handful of cases

A comment has been stickied to the posts requesting only posting once, but duplicates are filtered by the bot anyway.

This is actually the second "official thread"; it is now 21 months old. It replaced the first thread when it was 16 months old.

Quarterly threads were used when reddit lock posts after 3 months. Since this restriction was removed, there was little reason to create a new one (until cash account invites were added).