I don't know either how exactly to interpret the RAW_VALUE column (the big number in the final column of the line starting with "173 Wear_Leveling_Count"), but the most significant values in that line are columns VALUE and THRESH.
As long as VALUE stays well above THRESH, your disk should not be approaching its specified end of life yet. I believe the initial value on a fresh disk is 200. When it hits 100 (my value for THRESH), it will have performed as many erase cycles as guaranteed by the manufacturer. After that you're running on borrowed time. On my (relatively lightly used) MacBook Pro, VALUE is 0197, meaning I am perhaps 3% into the life of the drive.
For what it's worth, my RAW_VALUE is 12,891,062,312, a little lower than your value of 12,895,125,514 and about 1/1000 of 12,567,284,681,697 (VALUE: 093) and 13,031,154,781,293 (VALUE: 87) reported in one of the other answers.
For information, the file system seems have written 5.4 TB on my machine so far:
175 Host_Writes_MiB 0x0022 099 099 000 Old_age Always - 5445917
What matters for the actual life of the drive is not the data logically written by the file system but the actual internal write cycles, which can be significantly different. However, if what I have written so far is about 1/30 of its lifetime limit, that would be about 160 TB which sounds about right for a 256 GB drive like mine.
Best Answer
There's nothing in NVRAM or in the EFI firmware that make your 2011 MacBook Pro unbootable should you ever want to return to a previous macOS.
As I explained in another answer, NVRAM holds your preboot environment like boot volume. The worst thing that will happen is it will look for a bootable file where one no longer exists resulting in the infamous flashing question mark or prohibitory sign. Reinstallation of your OS will fix the problem.
As far as your EFI goes, it's equivalent to your BIOS, just much more advanced. There's nothing an OS installation is going to do that irreparably harms your EFI firmware.
Doing installations on separate SSDs is actually a good idea. I do that myself as SSD prices are now very low that it's more economical (in time) to just swap out a drive than it it is to format and install. The Macs (of varying vintages and models) don't complain at all. NVRAM is never reset or variables modified for any reason.
Now, for applicable models with the T2 Security Chip (the 2011 MBP is not), users have to be aware of Secure Boot. Basically, it will prevent unsigned operating systems from being installed or booted. However, that restriction can be turned off. It's basically the same thing as UFEI Secure Boot found on PCs.