How to detect end of Capacitor Pre-charge
A dead-simple approach would be to use a time delay relay rather than the MOSFET.

When power is turned on, the timer on the relay starts. When the timer runs out, the relay closes. Select delay time to be &gt;&gt; 5•RC and it should work fine. No need to actually sense the voltage if you know R and C. Not sensing the voltage also means not re-triggering under heavy loads.

If you love the MOSFET, you could, of course operate it with a one-shot timer, but then you need to find/make a source of power for it. A relay seems likely to be simpler and cheaper than the DC-DC converter.

A latching relay could be used with some simple control logic to smack it on after a delay and off when the main contactor is opened, so you don't need to provide it with power all the time (presuming battery == need to conserve power.) An extra pole on the relay can talk to the disable pin.