Link\Load Balancing Two ISPs with Authoritative DNS

Hello,

Before I go and invest in a Barracuda LB, I was wondering if anyone has/is using FG with Auth DNS - docs say it is NOT recommended to use Auth DNS (database size issue...?), but I need to have SOA DNS on premise in order to control inbound traffic (internet to local resources) and I would like it to work in case the main ISP connection dies, so DNS must be SOA in order to dispense the alternate A record to the world. 

I have an old Barracuda Link Balancer 330, which I can repurpose and put in-front of the FG, however this will definitely complicate the entire config and setup, and so I would like to avoid this if possible.

TIA,

/DL