"take down/take off/download or whatever " I love it! Download and upload are so meaningless most of the time! (Download means from the big computer in sky to your local computer, and vice-versa. But between local computers or between phones, the use is pretty fuzzy.)
Is it correct that you want to get all your pictures in one place? Are you sure that what you want to do with them you can do better outside of Photos?
You'll need plenty of disk space, since you will end up with multiple copies of pictures. You may want to use an external drive.
Checking: on your iMac in Photos>Preferences, "System Library" is grayed out, and "Copy Items" is checked--is that right?
And you have iCloud checked.
If you have "Optimize" checked, you should switch it to "Download Originals." Optimize doesn't guarantee that you have full images in you iMac Library, so you don't want that. If you change from Optimize to Download then it may take awhile for the full-sized pictures to "download" to your iMac. You can check on the progress at the bottom of the Library view. You can watch at iCloud.com
Then switch to your MacBookPro and set it the same way, making it the System Library. Only one Mac can be System Library at a time. Making your MacBookPro the System Library will "upload" its pictures to iCloud, and "download" the images it doesn't already have. You may then have a bunch of duplicates. iPhones & iPads connected to iCloud will have their pictures "upload" as well. So all your pictures will end up in iCloud and on your MacBookPro. Then you switch to the iMac, make its Library the System Library, and all the new pictures in iCloud will "download" to fill it.
If you want to put all of these pictures into a folder on your Mac, you can File>Export with the choice of "Export xx Photos," meaning the edited, keyworded, captioned, titled versions, or "Original Unmodified" pictures, meaning just like they came to Photos the very first time, without any improvements. You can download originals at iCloud.com, as well. You click on "Download" !
I have many thousands of pictures that I am gathering together and, without further complication, I think it's hard to beat Photos for organizing them. Sometimes I need a little help from additional apps, but Photos works well.
