Does Whonix offer me more security vs just using a middlebox if I’m only hosting hidden services behind the middlebox (not using other applications, etc)?
Yes.
How so?
Stream isolation, secure network time synchronization concept just to name two. See Anonymity Operating System Comparison - Whonix ™ vs Tails vs Tor Browser Bundle for more. Rather it’s scrutinized, well tested, documented and maintained.