Rod Elliott designed a power-on delay at http://sound.whsites.net/valves/ht-delay.html using the same trick as I described in a previous log at https://hackaday.io/project/11681-power-supply-power-on-sequencer/log/55676-latest-considerations

1. spy on the mains frequency
2. use the signal as input to a CMOS binary counter

Rod's binary counter is a CD4020 while I have 74HC4040 but the real difference is in getting the desired signal. He uses the secondary coil of a transformer while I don't have this luxury. The PSU is a small 1W AC/DC that uses a diode bridge at the input.

The solution I am considering now is an optocoupler powered by an old trick I saw in a magazine long ago.

The optocoupler's LED is usually in the 1.6V range, which is twice the voltage of two silicon diodes in series. The high-voltage diodes can be put in parallel of the opto's LED (be careful of the direction !!) and the whole is in series with the PSU's input.

The output requires a low-pass to remove anything above 50Hz (there can be nasty parasites around the zero-crossing zone) and then another RC network to control the /RESET of the counter...

(diagrams are sorely missing, I know)