Startup Failure MacBook Pro Mid-2012

The "I/O Errors" confirm my suspicion that your hard drive is most likely failing. There is a slim chance that the hard drive SATA Cable is bad (the cable on the 13" model is known to fail).

The first order of business is to stop using the hard drive as you are making the failure even worse with each attempt.

Remove the hard drive and install your SSD. Boot into Recovery Mode (Command + R) or Internet Recovery Mode (Command + Option + R) to boot to the online macOS installer. Sometimes it will only boot into the older installer for the OS which originally shipped on the laptop from the factory instead of booting into the current or latest installer. It is best to connect the Mac directly to the router using an Ethernet cable for a faster more reliable connection and to avoid any possible WiFi issues. Install macOS to the SSD. At least this should give you a working laptop.

I highly recommend that you have a professional data recovery service such as Drive Savers or Ontrack attempt to recover the data on the failing hard drive since you may only be given one chance at recovering your data. Drive failures always get worse with each attempt you make to boot the drive, mount the drive, repair the drive, attempt to transfer files, and even just having the drive powered on. If the drive failure is not too severe, then it may be possible to transfer some data using the Finder or better yet using Carbon Copy Cloner (CCC), but this is extremely risky and most likely will fail even with CCC's ability to ignore errors. The best way to even attempt to recover data is by performing a low level bit for bit clone of the failing hard drive to another drive of equal or larger size using a Linux command line utility (macOS interferes with this process) and is most likely what the professional data recovery service will do (although a professional also has more knowledge and many times will attempt to "fix" the drive if necessary (replacing parts or by using expensive software to turn off portions of the drive) to assist in a better chance of recovering the most data possible. Usually after this cloning process the failing hard drive will be dead. Keep in mind that data recovery apps are only made to recover accidentally deleted data or data on a corrupted file system. Data recovery apps are not designed to recover data from a failing hard drive that is producing lots of I/O Errors.

In the future make sure to always have frequent and regular backups of your computer and any external medial which contains important & unique data. It is impossible to recover accidentally deleted data from an SSD after the Trash has been emptied plus an SSD can fail at any time without any warning signs. With newer Macs the SSD is integrated into the Logic Board and is using hardware based encryption controlled by a proprietary Apple security chip. If the firmware on this security chip becomes corrupted (possibly through an OS update) or the chip, or Logic Board fail (or possibly any other part of one of these new Macs) you may permanently lose access to the data on the SSD where even a professional data recovery service will be unable to access the data.