Now that there is no startup sound, how do I know when to press a startup key combination?

What key combination? Safe Mode only requires that you hold the Shift key immediately after pushing the power button.

see > Use safe mode to isolate issues with your Mac - Apple Support

To start up in safe mode:

1. Start or restart your Mac, then immediately press and hold the Shift key. The Apple logo appears on your display. If you don't see the Apple logo, learn what to do.
2. Release the Shift key when you see the login window. If your startup disk is encrypted with FileVault, you might be asked to log in twice: once to unlock the startup disk, and again to log in to the Finder.

To leave safe mode, restart your Mac without pressing any keys during startup.

This is what works for me. Restart your iMac. When you see the screen go black, wait until you see the backlight illuminate the screen (not full illumination, just the backlight), then hold down the Shift key. Keep holding it down until you see the progress bar get about half way across, then release the Shift key. Wait until it finishes starting up. Look at the upper right of the screen. It should say "Safe Boot" in red letters. Maybe this will work for you.

Restart and immediately press and hold the shift key until you see the apple logo https://support.apple.com/en-gb/HT201262

Apologize if funny question. Is your iMac plugged into reliable power source, like UPS or uninterupted power supply, and the UPS is plugged into building outlets, with no wiring issues?

Immediately means as soon as you push the power button, the press the buttons. Should take less than 2 seconds to do this assuming you don't have any physical impairments.

Try plugging the charging cable into the keyboard first. Helps with my iMac Pro.