Please Help Me See the Big Picture - Is Photos Best Place for multi-decade photo collection

iCloud Photos is perfect for a single user with multiple Apple devices. It will help you to merge your libraries into one library and see the same phots and albums on all devices.

But it is not possible to share an iCloud Photos Library, unless you share the Apple ID, and that would be cumbersome.

You could share a joint photos library as an archive of all family photos in addition to your private iCloud Photos Library on an external volume.

I prefer it, if everyone has an own Photos Library. This way we do not have to fight over wich photos to keep and which to delete. A Photos Library is a very personal collection of the photos and videos I want to keep. And also our tastes differ, how the photos should be adjusted, and if they are a favourite photo or not. If everyone has an own Photos Library, you will have some duplicates on the computer, but that does not really matter. It is an additional backup of the the photos that everone likes and want to keep.

My 'Big Picture' question is how do I get my entire photo collection into the Cloud and still use Apple?

Adobe's Lightroom - the CC version - will do this, with the masters stored in the cloud, (and backed up to your machine or attached HD), with versions of the application available for Mac, Windows, iOS, AppleTV and other phone and OS systems as well.

I plan to keep my existing Photos libraries on my Mac and just use Backblaze to back the hard drive up into the cloud.

One snag with that plan: Photos Libraries backed up to BackBlaze are rarely recovered intact.

I know there is a path to import a Photos Library to Lightrrom Classic, but that's not the Cloud version. Not sure about LR, but I'm sure google will have an answer.

iCloud is a syncing service and not an online storage service. It's designed to let multiple devices which are logged into the same iCloud Account to have identical libraries.

First here are the things you'll have to consider before you begin with an iCloud Library:

1 - purchase enough iCloud Storage to house your entire photo collection, full sized.

2 - determine if you can house the full collection, full sized, on your iMac. You might need an external HD just for the library.

3 - the iPhone will have to have iCloud Library enabled and "Optimize Storage" enabled.

4 - both devices will have to be logged into the same Apple iCloud Account. WARNING: this may be a deal breaker as both devices would get the same emails, messages, etc. for that ID.

Do both you and your wife have user accounts on your iMac Pro? If so then an alternative method would be to put a communal library on an EHD that's formatted OS X Extended (journaled) with Ownership set to be ignored and directly connected by cable to the iMac Pro. This way both users would have access to it. New photos from iPhones could be downloaded to folder on the EHD using Image Capture and then imported the library. Then the folder deleted after the import was confirmed successful.

If you're interested in the second method let me know and I'll expand on how to combine what you currently have into one library for the EHD.

Disclosure: I'm not a fan if iCloud Library so would prefer the second method. But that's a personal thing for me. Others feel differently.

Nothing that special about Folders that contain other folders and albums... yes LR has them too. You can easily recreate your current set up there. But my job is not to sell you LR or anything else. Go to the Adobe website, read up on it, google around the internet there is a vast array of help videos and articles, review articles and so on, some made by Adobe, but most made by folks who actually use these apps everyday - far more than for Photos, for instance - and then download the free trial and kick the tyres on it for yourself.