Are there any steps I can take to make macOS use more of my RAM?
macOS will by default aggressively cache files, etc. in memory. In your case, it seems your programs simply aren't loading in that much data - and then there's really nothing more to cache.

If you keep the system running longer and look at the values again, you'll most probably see that the numbers have changed so that less memory is left idle.

You can selectively use more memory by adding a RAM disk to the system. I.e. a virtual "disk" that is really in RAM. You could then copy files to that disk to ensure that they stay in RAM. It is hugely impractical for ordinary desktop usage.