Using supercapacitors on DC-DC converter output for high in-rush current
Using a supercapacitor is reasonable. Just make sure your power supply doesn’t complain about:

charging current during the initial start-up
back-powering when PSU is off

Why the concern? The capacitors will discharge through the power supply when primary power is removed. Diode isolation can prevent this.
As far as charging current, a large supercap bank will demand much more current than the PSU can deliver, tripping the PSU overcurrent protect mode until the charging can finish.
In short, the cap bank needs to be treated like a battery, with current-limited charge control and back-power blocking to protect the PSU.