I have never used or heard of bitkey.io --- and I will preface that I am not expert by any means, I am simply relaying my experience in doing something similar to what you are doing. I will also say with the dozens that I have made that not one of the ones I have generated have been breached or had funds stolen.
I have created private keys using vanity gen on an airgapped pc that has never been online - downloaded the program to a usb drive and installed on the laptop.
can read about it and download it directly from here:
https://en.bitcoin.it/wiki/Vanitygen but that was also to create vanity addresses at the same time.
if you dont need vanity addresses, you can simply use bitcoin core to generate a new address and a new private key with it.
that is a very simple process to do in bitcoin core. or even in electrum if you do not want to download the all the block history.
as for the holograms and qr codes - I have done that to dozens of chips/coins. and as small or very close to what you are suggesting - I use a browser plug in on the same air gapped pc (yes it is a brave browser extension but does not need to be online) every program I install on the airgapped pc is done via a usb drive to convert those private keys into qr codes.
as for that "mini key" i think you are referring to "firstbits" that usually references the public address - can read more about it here.
https://en.bitcoin.it/wiki/Firstbitssee pictures for one of them I did for my personal vanity gen address -- the private key is a qr code under the holo on the rear, the paper in front of the slab has the public address as typed and as a qr code.
![](https://ip.bitcointalk.org/?u=https%3A%2F%2Fi.ibb.co%2FbWXpywp%2F100763947-250007466326654-4807290919420166144-n.jpg&t=665&c=ooKLr4eHhU8sGA)
![](https://ip.bitcointalk.org/?u=https%3A%2F%2Fi.ibb.co%2F71vZHGf%2Fslabbed-front.jpg&t=665&c=Bqflv-XNflmjpQ)
![](https://ip.bitcointalk.org/?u=https%3A%2F%2Fi.ibb.co%2FJdM7Rr6%2Fslabbed-rear.jpg&t=665&c=yEfsmtiZzqTh3g)
if you want more information - please feel free to pm me or to ask here.