Oh, also, as far as I know, I'm not using File Vault. Is that an external app, or something that is pre-installed on the Mac?
It's pre-installed. See the various links at this Apple Support page for more information. From your description of the symptoms, I'll go out on a limb and guess that the hard drive is failing. Hopefully, I'm wrong.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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