Firstly it's worth noting that if an SSD is going to fail as a result of you writing too much data to it, then this will happen (as a rough rule) around the one petabyte mark.
To achieve writing out one petabyte of data, the average user would need to run their computer 24x7 for somewhere between 10 and 20 years. In other words, you'll probably have replaced your entire computer before your SSD will fail due to too much usage.
That doesn't mean your SSD won't ever die (that would be a silly thing to say), but when it does, it'll probably be due to something else.
How Long Do Solid State Drives Really Last? | How To Geek
Regarding what to backup, you don't need to backup both drives. In fact, I'd argue that a backup of the operating system isn't particularly useful. Put it this way, if your SSD blows up which will you complain the most about - losing Windows or losing 10 years worth of documents? Worse case is you have to buy another version of Windows, but you won't get your documents back.
On that basis, I'd backup the following on your C drive:
- Licence keys for software (look for ProduKey, KeyFinder)
- Drivers (look for Double Driver)
- Any important documents you've stored there
and I'd backup the entirety of your D drive. If this isn't possible, then I would select the key folders that contain files that would upset you if you lost them.
Finally, remember that the best backup solution follows the 3-2-1 rule. That is 3 total copies of your data, 2 of which are local but on different mediums (read: devices) and at least 1 copy offsite.
The 3-2-1 Backup Strategy | backblaze.com