Right now, the Sleep Current is between 0.7 to 1.0 mA for the Boron LTE (at least the last firmware I tested).
So, 0.001 Amps * 24 hours/day * 365 Days/year = 8.76 Ah per year, just for Sleep.
As you said, your battery is rated at 13 Ah, so 5 years wont happen with 1-cell.
If you use external circuitry for the Boron’s EN Pin and to catch your Interrupts, you will reduce that by a factor of 10. That’s likely your best chance to reach your 5-year Target.
Even though the Fuel Gauge wont be correct for a Li-SOCl2, the Boron can still accurately report the Cell Voltage. A simple equation based on your Battery’s datasheet would be used to calculate the remaining “life” of the primary cell.