Mac mini (2018) startup after software update. I have to switch it off and on again.

1) Try starting up in Safe Mode followed by a normal Restart first.

see > Use safe mode on your Mac - Apple Support

2) Try disconnecting any and all external drives and test the startup.

If that helps, reconnect one drive at a time and re-test startup.

3) If you do end up reinstalling the macOS.

Make sure that you have a good backup set aside.

Then reinstall over the top instead of erasing and reinstalling.

Boot into Restore mode,CMD+r keys...

Look at top of screen for Utilities menu...

In Disk Utility>View, select Show all Devices, highlight the top left entry.

Run Disk First Aid on all items in the left panel, from top down.

Are you running any VPN, Anti-Virus, or Cleaning apps?

We need to see what all is running, a report from this will not display any personal info...

Using EtreCheck - Apple Community

EtreCheck is a FREE simple little diagnostic tool to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info. Give it Full Disk Access.

https://www.etrecheck.com/

Give etrecheck Full Disk Access before running.

Thanks for Old Toad’s etrecheck instructions…

Slow iMac 2017 - Apple Community

rLo_101 wrote:

I tried a safe mode restart, which was a useful tip thanks, but it’s still the same.

You're welcome.

There are no drives connected and the power supply is O.K. 

That question has to be asked.

Once on I can Shut Down and Startup as normal, it only happens after it’s been turned off for several hours, over-night, which means waiting until the next day to test any fixes.

In more detail, please explain the steps that you take when shutting down for several hours and over-night.

I was invited to enable FileVault as part of the install.

That was an optional invitation.

rLo_101 wrote:

It's working normally now after the Disk First Aid

That is good news.

but if the power supply is a factor, there's a USB hub built into the monitor with its own connecting USB cable from the Mac mini USB port.

Not sure why that would matter..?

It's probably better practice to let this type of monitor warm up a bit rather than trying to turn both on together in quick succession.

My 2018 Mac mini with dual displays stays connected to the power.

It only gets shutdown if I'm away for more than a couple of days.

The displays go into standby/sleep mode when it is not in use.

It has been used this way for almost 6 years without issue.

Runs the latest macOS 15 Sequoia without any issues.

It is not wise or necessary to install and run third-party Anti-Virus, Anti-Malware or Cleaning junk-ware on a Mac.

In fact, most will do nothing except slowdown and make your Mac unstable, or worse inoperative.

Follow the developers directions and uninstall the third-party junk-ware.

Then re-run Disk Utility First Aid again.

• Yes, anti-malware installed.

Which one?

There is no reason to switch off the power at the wall socket. In fact, it can cause startup and other issues if the Mac mini's PRAM battery is low or dead.

Has it spent years setting without AC power?