So after much research, and reading reviews of both Ubiquiti and MikroTik gear, I have determined that Untangle CAN actually work properly with a PPPoE connection on its WAN port.  There is documentation that says otherwise, but apparently this relates to an older version of Untangle (pre V10) and all new versions now work correctly with a PPPoE interface.  Plus the modules that where affected where ones I wouldn't use - WAN Balancer and WAN Failover.

So I am going to go with just the Untangle server direct to the ONT.  I have a dual port Intel Gigabit NIC in the server, so speeds should not be an issue.  And hopefully being a 'full PC' and having used Untangle for years, it will be easy to set up.

I will post back when the install is done.