Is it worth merging with another FC? Or what to do with a dead and empty FC?

[Jynx]

Also, would it be fair to have a requirement for members to join the forum I have created for the group, or is that overkill for a social / casual FC?

I'd say overkill. Stick with a discord server and don't be pushy. Social/Casual people don't tend to enjoy being told what to do.