Ok, Here's what I did to solve it! (Your milage may vary). FWIW - tool dozens of hours to troubleshoot / fix with 50k+ files.
Best I can tell, the analysis gets stuck or won't start until uploaded to iCloud is complete. Here's what I did to fix:
1) remove non-compatible files. For me I had about 30 photoshop files (.psd) that were holding up the process. Use Smart Folders to identify and then export and delete. Other threads have the list of file types.
2) remove corrupted files. this one is harder, as I had 50k+ files. The problem files were mostly movies. And most problems were slow motion. Use smart albums to filter and then export. There was one .mov fie that would crash photos if opened. Hard to spot, but it was blurry in preview and/or colors off in preview
3) *duplicates*! I have a large number of dups from prior imports and mergers. Most are OK , but some causing problems. To solve:
- check if the number of items on local cpu are same as iCloud. For me, there were more local than in iCloud.
- Then identify those that are not uploaded. (Smart Folders for "unable to upload" won't work!)
- For each year (or month) identify number of pics/mov - manually scan if there are more local than iCloud. Find the duplicates and then delete (or export) from local. All of mine were associated with HDR where the camera also captured raw or jpeg.
- Repeat for all years/months (I had once cpu w/ download originals, and one with iCloud photos optimize to compare)
- once number of pics locally = number of pics on iCloud; the analysis proceeded ok!
All in all, was a pain in the A** but it does work now