Will this work - Maybe?
Been thinking for some time of the prudence of getting my critical Aperture library migrated to something I can use into the far future to access the roughly 145,000 images amounting to over 5 TB in storage requirement, while having the basic organizational structure as in Aperture. I have already had to switch to Lightroom to handle all my new photos because many of them are being shot with my Nikon D850 (hoping to find Luminar's DAM superior if they ever get it out.) Had settled on checking using Photos to do this, but had not tried that due to the size of the task.
Yesterday I was checking the contents of several older SATA drives, and ran across on a 750 GB drive an old Aperture library from 2010 that was only about 525 GB in size with about 35,000 images. Decided to open it with Photos (without bothering to first open with Aperture and upgrade) to see what would happen. Seemed to take forever to get anywhere, and then seemed to stick on 34% done for another forever. Then all at once photos started to appear as thumbnails were built in a Photos display window. All the photos seemed to be present, and in a structure similar (maybe identical) to that in my continuation Aperture library up through the applicable date.
The resulting volume now has both an Aperture Library, and a Photos Library package. The latter is slightly larger when checked than the Aperture Library, but it is one of those Apple magic tricks has both packages sharing many of the same subfolders, and the total data on this volume (exclusively the A Library and derivative) is only a bit more than the size reported for the Photos Library.
So I am at present letting SuperDuper copy the nearly 6 TB ALib from its 6 TB drive to a new 8 TB USB3 drive, and will subsequently try to open that A Lib in Photos hoping there will be enough room to allow the conversion. From experience I know this first step will take over 36 hours (maybe more) and then it will take some unknown time to then try to open in Photos.
Btw, this huge Aperture Library, while still working in Aperture, in 10.13.4, has gotten very sluggish to access. With smaller libraries Aperture seems to be working at a normal speed.
Ernie