How can I completely wipe my 2017 iMac before disposal?

See:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Of course a hammer on the HDD / SSHD can be a pretty effective too.

I’ve “mechanically destroyed” more than a few HDD platters over the decades.

Chattanoogan wrote:

Re: “… The Apple instructions … assume the use of an SSD or assumes Filevault …”

Thanks for this significant caveat.

You're welcome.

Besides the Disk Utility secure erase option, there is a third party open source option which should be easy for people if they cannot boot to Recovery Mode or a bootable macOS USB installer. ShredOS is a good option for the older Intel Macs since a bootable ShredOS USB stick can be created using any computer or OS that can access the Internet.

https://github.com/PartialVolume/shredos.x86_64

Use the downloaded "ShredOS .img x86_64bit for USB Vanilla" or "ShredOS .img x86_64bit for USB Vanilla DRM" as a source for Etcher (Mac, Windows, Linux) which will create a bootable ShredOS USB stick. I don't know why the ShredOS FAQ doesn't mention this option for macOS users although they do for Windows' users.

Option Boot the Intel Mac by holding the Option key immediately after the startup chime. Select the orange icon which will most likely be labeled "EFI".

This option should only be used if the user does not care about reinstalling macOS afterwards, or the user has already tested a bootable macOS USB installer.

A 2017 iMac cannot run macOS 10.10 Yosemite. A 2014 and Early & Mid 2015 iMacs did ship from the factory with macOS 10.10 Yosemite.

If your iMac uses a hard drive, then you will want to perform a secure erase on the hard drive by writing zeroes to the entire hard drive in order to destroy your personal data. A simple erase will allow a data recovery program to potentially recover data from the drive if only a simple erase is performed with Disk Utility on a hard drive. Disk Utility has an option to secure erase a hard drive. As long as the hard drive is not failing too severely, then this should work.

If your iMac has a Fusion Drive, then things get a bit tricky since a Fusion Drive combines an SSD and a hard drive together. You would still want to perform a secure erase of the hard drive portion of the Fusion Drive. The SSD portion just needs a simple erase since SSDs work differently and would destroy all data on the SSD with a simple erase. After erasing the two drives separately (the hard drive with a secure erase), then you would need to recreate the Fusion Drive setup prior to reinstalling macOS:

How to fix a split Fusion Drive - Apple Support

If your iMac only has an internal SSD, then the directions @Chattanoogan provided is sufficient for destroying any personal data stored on the SSD.

It is really unfortunate that the Apple instructions neglect to consider older Macs where a hard drive may be involved. The Apple instructions linked by @Chattanoogan assume the use of an SSD or assumes Filevault has been enabled on the older Macs (2017 & earlier).

I highly recommend you first create a bootable macOS USB installer while you still can since many people experience problems using Internet Recovery Mode.

 1. Insert the Mac OS X Install disc that came with your computer, then restart the computer while holding the C key.

  2. When your computer finishes starting up from the disc, choose Disk Utility from the Installer menu. (In Mac OS X 10.4 or later, you must select your language first.)

   *Important: Do not click Continue in the first screen of the Installer. If you do, you must restart from the disc again to access Disk Utility.*

  3. Click the Erase tab.

  4. Click the disclosure triangle to the left of the hard drive icon to display the names of your hard disk volumes and partitions.

  5. Select your Mac OS X volume.

  6. Highlight the drive, select Partition Tab, then Format type... MacOS Extended Journalled, select the Security Options button, choose Zero Out Data, Erase... after completion do a new install.

Or...

Open System Preferences>Accounts, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

In the same pref pane highlight your old account, click the little minus icon, then use Disk Utility to Secure Erase Free Space.

Re: “… The Apple instructions … assume the use of an SSD or assumes Filevault …”

Thanks for this significant caveat.