Lost Disk Space after Uninstalling Windows 10

I recently used Boot Camp to install Windows 10 on my iMac. Something went wrong with the installation, and it was not working correctly, so I decided to uninstall and try again. I used Boot Camp to uninstall, and it said that it would restore my Mac HD to the full terabyte. I assumed that it would actually do this, but apparently that was silly of me. When I went to reinstall Windows, it was showing that I only had the space that was on the macOS partition when I'd had Windows installed. Disk Utility shows the rest as "Free space."

Unfortunately, I can not drag the resize controller as it suggests. I can only drag to make Mac HD smaller. If I select the free space and decrease the number of partitions, it shows that it will combine it with the Mac HD partition, but when I actually go to do it, I get this:

This is what I get from diskutil list:

I booted with Command + R and deleted the Mac HD partition, hoping that it would combine the two, but it did not. So now I have a fresh install, but I still only have the 491/1000GB.

I've seen many threads on this topic, but I've yet to find any actual answers. I'm generally a PC person - this is my work computer - and this is boggling my mind because I didn't think allocating space could be so difficult! Like I said, I've already backed up and erased the hard drive, so at this point I'm willing to try anything.