FCP won't stop copying files

"Now there's also a line of dots appearing above the last part of the sequence as if it's loading."

If that's in the timeline, that simply means that section would playback easier if rendered, it has nothing to do with anything "loading".

Look at those clips in the Browser, be sure there's no symbols or indicators on them, showing they're still copying over into the Library.

What you created is called an "image sequence" technically, and it's a LOT of images, and that many usually take a bit of time to copy over. But if you let it sit for an hour, I would assume it's done.

But my bet is on the size of those JPEG files. Image Sequences are usually made with very, very, very large sized images. Be sure all those images are resized to the correct frame size of your timeline, and at 72 dpi before bringing them into FCPX. Over sized image files can give FCPX fits, much less an entire image sequence of them.

If the library is stored on the external drive, how is drive formatted? ProApps require HFS+ or APFS.

Format storage devices for Final Cut Pro, Motion, and Compressor - Apple Support

Press Command-9 to open the Background Tasks window. Is something still in progress there?

the sequence I entirely on an external Hard Drive, none of the images are on my Mac.

Q1: Are the original media (still images/videos/music files, etc.) stored on an external hard disk?

Q2: Where did you save your library? MacBookPro or external hard disk?

Q3: Where did you specify the export location? MacBookPro or external hard disk?

Q4: Please show me the media loading settings. Open the library. →⌘I →shift+⌘3 to take a screenshot (it will be saved on your desktop). →Post a screenshot from the image icon.

Ben, I totally agree with everything you said, except… 72 dpi or not should be irrelevant, the frame size is what matters. A 1920x1080 image should work identically, irrespective of what dpi is set in its metadata.