Only import them manually into the Photos app (the iCloud Photos option must be turned on).
Just drag and drop a folder with photos and videos to “My Albums” in the sidebar. Or use File > Import, and select the Keep Folder Organization checkbox.
Most importantly, if you don’t have enough space on your internal drive, move the Photos Library to any external drive first.
P. S. I've already successfully used this technique on hundreds of gigs. Then I turned on the Optimize Mac Storage option to save space on Mac.