The Routerbox 750 has arrived, and is now configured. The packets that comprise this post are flowing through it now 
It’s different enough from anything I’ve seen before that the specifics of it threw me for a bit, but it’s all configured and works. And it’s way faster than the old firebrick was – I’m seeing webpages load a lot faster than they did before, it’s actually a very noticeable speedup.
The only thing I need to say to anyone getting one is that the default gateway is a pig to make work – it’s not just good enough to put in the IP address, you have to also populate the interface too. But that done, it’ll work like a champ.
I’m a happy camper