Meg, thank you so much for your reply.
That certainly seems to be the correct solution... however applying that HDR Tools effect only seemed to take my videos from absurdly overexposed to just very, very overexposed. Not sure what that is, but...
What seems to be working for me, here, now is simply waiting for the videos to upload from my phone to iCloud and then exporting them to the Finder out of Photos (and then bringing those into FCPX). Photos seems to make the conversion automatically, similar to when it exports an HEIC photo as a JPEG. (What I had done earlier, to minimize the hit on my data cap from the above uploading/downloading steps while traveling, was to pull the (obviously unconverted HDR) files straight off my phone using Image Capture.)
But thank you again for being so willing to help.