In my experience, there's little benefit to trying to "optimise" a physical HDD.
These days, we have so many gigantic files, and our HDDs are so gigantic too, that it's processing the data that takes time rather than accessing it and reading it off the HDD.
The only way you're going to improve your situation - given that the files on your current logical drives are, apparently, enough to slow things down - is by purchasing a new HDD.
Ideally, maybe go for a 256gb SSD (pretty cheap these days) and install your OS and some other frequently used software on that then use your current HDD as a dedicated file storage drive.
Personally, I have a 256gb SSD as my C drive, with my OS on it, my D drive is a 2tb HDD which I install software on and my E drive is another 2tb HDD which I store files and other junk on.
To reitterate, though, I wouldn't bother wasting time juggling around files or trying to modify logical drives.
If that amount of files on a single physical HDD is causing it to run slowly, monkeying around with "optimisation" tools isn't going to help much and, at worst, could ruin your day.