Originally Posted by texas_nightowl
I think I remember it being a thing in the past that you left some unallocated space that if Windows detects problems with spaces on the drive it could use some of the unallocated space. But I may be totally making that up too.
Dunno about that, but the advice given was for overprovisioning. Long story short, erasing flash memory is a relatively slow process, so if an SSD is completely full it can slow down when data is overwritten. However, SSDs already have some spare area built-in. Your 256GB SSD in actuality has ~275GB of memory in the flash chips. (Long story short, 275GB using the base 10 prefix is about equal to 256GiB using the binary prefix. G = 10^9 while Gi = 2^30)
EaseUS Partition Master
has worked for me in the past, but I'm not entirely sure the newest free version works or if it just prompts you to buy it after playing with settings. At the very least you should be able to stick the System Reserved partition on the other side of the unallocated space and then expand your C:\ partition into the free space.