How do I unlink my Mac mini synced to my iPhone and iPad?

Easy, if you intend to still use the MM and just want to share it with him, then create a new user for him. Apple explains this in New user or group setup on Mac - Apple Support (BW)

However if you are gifting the Mac to him and he will be the sole user, then please follow the steps Apple outlines in What to do before you sell, give away, or trade in your Mac

Kindle synchronizes access to your purchases across all devices that are running the Kindle app, and logged into the same Amazon account. I believe that each device has its own set of downloaded books, and its own cache of Kindle data.

On the Mac, this is in a local storage area that is not synchronized with iCloud. So I'm surprised that you would get any sort of message referring specifically to there not being enough iCloud storage to read Kindle e-books.

lanstrad1 wrote:

Older purchases will still be usable, but as long as there is no legislation to stop that battle, you may not be able to purchase media content from Amazon/Kindle from an Apple device.

You can purchase Kindle books from Apple devices. You just can't do it through the Kindle app. But you can do it through any browser.

fruity55 Said:

“Giving Mini to my husband: […]It is saying there is not enough I cloud memory to read books.[…]”

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Can you provide a screenshot of this?: Take a Screenshot on your iPad - Apple Support

Clearing Space:

As for clearing space, here is how to go about doing so safely, without the loss of data: Getting a Quick and Direct "iCloud Drive-to-External Hard Drive" Download - Apple Community

When you said that you have a "Mac mini" – did you mean to say that the old system was an iPad mini?

People don't normally refer to Mac minis by generations ("2nd generation"). The second Mac mini that Apple released was the Mac mini (Late 2005). It can't run anything later than Leopard – yet a Mac would need to be running El Capitan or later to use iCloud. This, and your tag line, suggest that maybe the old Apple device that you're giving to your husband is a 2nd generation or 3rd generation iPad mini.

If it is an iPad, and you are planning to transfer it solely to him, see:

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

Then have him set. it up with his own Apple ID and his own Amazon ID / Kindle account.

fruity55 Said:

"How do I unlink my Mac mini synced to my iPhone and iPad?"

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Unpairing Mac from iPhone an iPad:

Seems to be plenty ways of performing this unpair. See the tips mentioned here: How to Disconnect Your iPhone From a Mac: 8 Ways - MUO

I would be careful with using an apple device in regards to future kindle purchases

indeed, we are all trapped into a battle right now between amazon, apple and Google Playstore, as no one wants to pay dues to the other in regards to new purchases. Users are being kept in hostage in this battle.

Older purchases will still be usable, but as long as there is no legislation to stop that battle, you may not be able to purchase media content from Amazon/Kindle from an Apple device.