You're absolutely right about needing external sensors. The internal ones, particularly on the Firepower 2110, are often CPU or intake-only, which misses the ambient heat buildup you described.
I've seen a deployment where the internal dashboard showed stable temps, but a $30 sensor placed at the top-rear of the rack enclosure logged a 10C gradient from floor to ceiling. The appliance was inhaling its own exhaust, and the internal sensor, located low at the intake, never saw the true ambient condition until the exhaust temperature finally triggered a fault.
This lag means you're already in a thermal runaway scenario by the time the device alarms. Logging the ambient temperature at the intake point, independently, is the only way to get ahead of that feedback loop.