Or you could simply redistribute the space between your C and D drives by repartitioning. It's pretty simple, usually just a matter of dragging a line across the screen and being patient. A free program like the one here https://www.paragon-software.com/home/pm-express/ will lead you painlessly through the process. You can gain another 200 GB for C:\ and be sure that you have not mistakenly got rid of something you wanted, or moved it to a spot where the software using it can no longer find it. I do this on all my PCs before I start filling up the C:\ drive, leaving 10-20 GB on D:\ which seems to be just fine, and is still probably overkill. Certainly I have never had any problem about lack of space on the D:\ drive because I never use it - like you I use external USB drives for the important stuff. They get hidden in the loft when we go away, and should the PC be stolen I won't lose my photos or music.
Many PCs seem to come with a D:\ partition taking up half of their hard disk, and unlike you many users barely know how to use My Computer to look at what they have got. Such PCs are often junked because they keep telling the user that there isn't much room on C:\, and the user has more money than sense. You have to wonder if this is a marketing trick...
I'm still wondering why no-one has suggested this solution. Yes, it's a good idea to tidy up your files as well and you will gain some space, though seldom as much as the utilities promise.
Also, a large Outlook email file just slows down Outlook. Check out the Outlook help. You can archive the current file on a USB stick or one of your other USB disks if you want to keep all your old email for reference, then delete all emails older than X days or weeks. You could just start another email file, but if you have built up a complicated folder structure you probably want to keep that and not have to rebuild it.