a lot of those tweaks do nothing since windows7 already takes care of it(i'm looking at you superfetch/prefetch). there are articles pointing them out as myths with microsoft engineers as references.
Not saying all are bad, there are some good tweaks, but most are myths, and others go against the benefits of the SSD because people are afraid of "using it up".
e.g. I should want my browser caches to be stored on SSD, cause that's actually stuff that I want sped up, not slow. The tweak to move that is misguided.
That being said. if Space is your issue, then first find that space, since that seems to be your main issue.
To regain space, most likely you need to specifically get rid of:
1) hibernate file for hibernate or hybrid sleep (regular sleep still works)
2) page file for virtual memory (set to something small, or disable...).
Just look over your C:\ drive and report back where all your space is going. Add it up by going through each of your folders and see if it matches with your C:\ total
That is your MAIN issue.
The stuff about optimizing for performance is a lesser issue
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For your main question. Why should you have SSD?
It's to make your computer faster.
No other thing will speed up "productivity" compared to increasing storage speed. Most delays are waiting on I/O these days and not CPU.
Why would you have the justification spend extra $100 for cpu upgrades which gets you maybe600mhz faster. Straight math should tell you this is maybe a 15% increase.
Comparatively spending $100 on SSD which will get you 100% increases in real-world activities.
Part of your problem, is you made a mistake in buying perhaps too small of an SSD, so you're left to micromanage space to make it fit, and SSDs lose speed the more full they are.
If you had like a 128gb ssd, you would get pretty much all the speed increases and not have to worry about anything or worry about having to tweak. This includes using sleep/hibernate and everything the same way. (but if you do some tweaks you could go even faster or recover some space.
If you go along the thought process, Well I can wait the extra second or 2 seconds so why do i need it.
Then you end up realizing that your computer from 10years ago was good "enough" to do what you want it to do too, so why aren't you using your 10year old computer.