MacBook Pro 13" 2012 Error Report every time I start up

While it can take a while for the long/extended selftest to finish even on an SSD, the selftest should finish as long as macOS is not interfering. I would suggest replacing the hard drive SATA cable since the cable in the 13" MBPro (mid-2012) model has an extremely high rate of failure especially when using an SSD.

If the hard drive SATA cable is not the issue, then I would reset the SSD to factory defaults by using the SSD's built-in hardware secure erase feature. Besides restoring lost performance the hardware secure erase will also reset the SSD to factory defaults which can sometimes fix an SSD if it isn't working correctly. To use the built-in hardware secure erase feature of the SSD requires using a Linux boot disk & utility. Parted Magic can be used to perform the hardware secure erase on the SSD. The last free version of Parted Magic is available from the MajorGeeks' link here. Here is information about the hardware secure erase feature for the newer paid version of Parted Magic. While the app interface looks different, the basic procedure is still the same. While the app has several ways to zero a drive only the "ATA Secure Erase" option will reset the SSD to factory defaults. The correct option will have you sleep & wake the laptop as well as ask for a temporary password for the secure erase so use a very simple password such as "abc". You will need this password to disable the SSD's security feature if the secure erase fails or is interrupted otherwise the SSD will be a brick.

https://partedmagic.com/secure-erase/

Also check to make sure the SSD's firmware is up to date.

Uncheck "Put hard drive to sleep when possible" in the Energy Saver System Preferences. Some SSDs have trouble waking from sleep mode.

What is the make & model of the memory upgrade? Macs can be very picky about the memory used so most contributors will only recommend memory from Crucial or OWC.

Run the Apple Diagnostics.

Run DriveDX and EtreCheck and post both reports here using the "Additional Text" icon which looks like a piece of paper.

Did you replace the hard drive SATA cable when you upgraded to an SSD? The hard drive SATA cable on this particular model has an extremely high rate of failure.

I don't see anything in the reports either.

You can try running the SSD's internal self diagnostic test. If you only have the trial version of DriveDX you cannot run the long/extended selftest using the GUI, but it is possible to initiate the test from the Terminal app using the command line utility. Assuming DriveDX is still located in your Downloads folder run the following command to initiate the long/extended selftest. You should be able to view the results in the DriveDX app itself.

sudo  ~/Downloads/DriveDx.app/Contents/Resources/smartctl  -t  long   /dev/disk0

When you execute the command it should provide you an estimate of how long it will take to complete. The more the SSD is being used the longer it will take. While you should be able to review the results of the test in the DriveDX app itself, here is how to access the results from the command line (it may not show any progress or results until it has finished):

sudo  ~/Downloads/DriveDx.app/Contents/Resources/smartctl  -l  selftest   /dev/disk0

Note: The "-l" is a hyphen followed immediately by a lowercase letter "L".

The Apple Diagnostics may fail the battery test when a third party battery is used and is not unusual.

Make the change I suggested to prevent the SSD from being put to sleep to see if it solves the problem.