PHOTOS: How to make an intelligent backup from multiple sources?

1. Get all the images into one place and managed by one app.
2. Back that up.

As you're posting on the Photos forum, and as that app is on your Mac anyway, then I suggest you use it. First gather all the images onto - say - an external HD. Then import them to a Photos library on that HD. It will detect most duplicates as you try and import them. Any others you can get with the help of PowerPhotos:

https://www.fatcatsoftware.com/powerphotos/

Once that's done and they are all in a Photos Library, then you'll need another disk for back up. As for an app to do it: Time machine is on your Mac, but others like SuperDuper or CarbonCopyCloner are equally good.

Google Photos is an excellent additional backup to the cloud.

With Photos you can keep the folder organization that you had in the Finder:

So you can keep the same folder organization you had in the Finder and add keywords, titles and other metadata to help organize the photos to make them easy to find.

If you click on the arrow next to the "iPhoto Events" folder you'll see all of your Events from iPhoto as Albums.

Not an endless repository of ideas here. What app do you want to use to organise them? You choose. But the principle remains the same. Gather the images together. Root out the duplicates. back them up.

I've not found any other alternatives that I'd be prepared to trust.

That's a mite harsh. Viable and perfectly reliable photo managers include

Mylio - consumer level app, think of it as iPhoto+

Lightroom - consumer level app, think of it as Photos+

Lightroom Classic - Pro Level app. Quite the industry standard

CaptureOne - Pro level app

Exposure x6 - Pro level app

On1 Photo Raw Pro-level app

All of the, like iPhoto, are both managers and parametric editors

PhotoSupreme is a photo manager only

There are other apps that are very good Raw processors, but quite limited as managers, such as DxO Photolab, and some that are very well advertised but not so good in practice such as the Luminar range.

I've used or tested all of these apps at some stage.

You list a lot of back up applications. But the first question is how do you organise the images when you gather them? The Finder? Photos? Some other app?

As for the listed applications: Carbon Copy Cloner is an excellent app for backing up to a hard disk.

Google Photo is a service that back up your images to the Cloud.

Which do you want to do?

Hi

There is no app that will let you keep everything exactly as it was in iPhoto. As Old Toad has pointed out, Photos will put events into folders, but that is far as it goes. Any non apple app won't even do that.

Yer_Man has told you what you need to do. Gather the images. Decide what system you want to put them in, and start organising.

Photos has the advantage that you already have it, and it is free. The space issue can be resolved by storing your library on an external disk (But there are requirements on how to do that, so come back if you decide to go that way)

Alternatives:

Just use a folder structure and browse using finder - not very convenient for a large image library.

Adobe Lighroom - monthly cost.

I've not found any other alternatives that I'd be prepared to trust. Certainly not google photos - and even that has a monthly cost now if you don't want degraded image quality.

Having said that - here is an "11 of the best" article for you to read through. You'd have to make your own mind up, and probably many are not available on MAC

https://www.adamenfroy.com/best-photo-management-software

Note: Carbon Copy Cloner isn't image management software, it is disc copying and backup software - a completely different beast.

I have two backups. One with Tine Machine to backup the Photos library along with the rest of the boot drive and the second on a clone of my boot drive using Carbon Copy Cloner.