Oh, that's interesting. I've been wanting to use a custom domain just for forwarding, and the enhanced personal plan seemed like overkill. I'll have to look into the limitations of the lite family plan.
Uh, it says "Use your own domain or one of ours" at the top. That sounds to me like you can use a custom domain with any family plan, Lite and Full included. Are you sure that isn't the case?
https://www.fastmail.com/signup/family.html