By todays standards I'd say a J1900 is a pocket calculator and its not Zens fault FreeBSD has a horrible PPP implementation.
I'd absolutely love to be done with PPP, but most ISPs use it because it ties into their legacy accounting system which might date back to dialup days. When even A&A use it (allowing them to offer L2TP), I don't think you can call out Zen for doing so.
I get the principle, without PPP it would still be enough for a basic router, but to suggest you need a beefy CPU seems a bit of an exaggeration, considering what a beefy CPU is today. Plus if you're doing so little you aren't taxing a J1900, do you even NEED pfSense?
Even if Zen was DHCP, I have AAISP L2TP and several OpenVPN instances on my box, so a J1900 would have a very bad time.