Are you using
iPhoto ? or are you using Photos (which version?)
I have used iPhoto since first introduced in 2002 and have stored all my digital pictures since then (had a nikon camera back then) and recently began using my iPhone more than my cannon and nikon cameras
iPhoto no longer works on Catalina and iPhoto has been replaced by Photos since 2014 - that said they are similar and the iPhoto library can be upgraded (make a back up first) to Photos (versions 1 to 6)
I have always used iPhoto and most recently Photos 6 as the hub and central storage for all my digital photos and pictures - once they are in the Photos library I delete the original from the source and use the Photos app to organize all my digital images - also I download all my pictures from my phones- cameras and iPad into Photos and delete them from the devices
I have done this for almost 20 years since my first digital camera (Nikon) and have now split the libraries into 3 separate large libraries (large to me) - 1)Family and personal, 2) Work Images, 3) Hiking Pictures - all 3 libaries are larger than 150g = 500g of images and some movies - and very easy to back up these Libraries weekly with CCC - 5 min to back up
I like iCloud - but I would never trust my pictures and memories on iCloud - if your apple id does not work you are locked out of your pictures - IMHO