Syncing A New Photo Library

Hi and sorry if this is a bit long! Any help / advice would be SO appreciated.

I've been trying to update and combine x3 Photo Libraries on my MacBook Pro 13" 500gb (Early 2015), but I'm having trouble getting iCloud to sync the new Photo Library I've created.

I had a Photo Library (2017-2021) as the Source Library on my MacBook Pro which was synced with iCloud and working absolutely fine, but I didn't have enough storage to "Download originals to Mac", and also wanted to combine this library with two older libraries stored on an external HD (2004-2012 and 2013-2016).

I managed to clear my Mac HD enough to download the 2017-2021 originals to my Mac, then move it to the external HD with my other two libraries. I then used PowerPhotos to successfully combine all 3 libraries as a brand new library (in hindsight, I should have merged the older two into my current one, but for some reason, like an idiot, I didn't).

I now have a downloaded 2004-2021 Library on an external HD (what I wanted), and would like to use this as the Source Library, and sync it to iCloud (I'm happy to have low-res images on my iPhone / iPad, but would prefer to access high-definition photos from the external HD when viewing on my Mac. It also ensures I have a fully backed up copy of the photos as well as stored on iCloud).

I clicked on the "Download originals To Mac" and ticked the "iCloud Photos" box, and a large countdown started saying it was syncing photos to iCloud (26,000+ photos and videos) and slowly decreased to 13,000 photos (ish) over about a week. It didn't make it the rest of the way, but seemed to get stuck. There was some sort of error (I can't remember exactly what it was now, which I seemed to sort out), then it then switched to "Restoring from iCloud" at the bottom of the screen and has been stuck ever since. It's been about 3 days on this now.

On iCloud.com all my combined album folders for 2004-2021 are there (with the 2017-2021 folders duplicated because of the creation of a new library 🤦‍♂️), but only a couple of photos have appeared in the duplicate folders / albums. A fair few random older photos have been downloaded at the start of the stream, but not all of them, and they are not appearing in the designated albums yet either. For the last few days, absolutely nothing has appeared on iCloud.com apart from any photos / videos I take currently (it's updating as normal).

I don't mind having everything wiped and just re-upload my main Library again (I'm happy to wait), but I wouldn't know how to safely do that. I'm a bit concerned I've messed things up by trying to sync a whole new library to iCloud almost over the top of my old one!

I've also tried "Repairing" the new 2004-2021 Library, but this hasn't changed anything (it errored halfway through the first time I did it saying that it was corrupt, but did a successful repair after I restarted my Mac).

I have considered trying to re-sync my 2017-2021 Library with iCloud again, then combine the old libraries with this again and try once more, but don't want to create a load of doubles or overwrite all my photos somehow.

Can anyone suggest what I should do?

Thanks so much!