Stop using Photos. There is no reason to use that lacking piece of software when you have LR.
What I believe is being uploaded to the cloud (iCloud) are reduced JPG files.
If Photos is anything like the older iPhoto when you import images into it they get copied into the Photos folder structure buried deep in a Package file,folder (a type of folder where subfolders are not easily accessed by the user).
Apple likes to HIDE things from the user. Including their original image files.
Could you recommend a backup service other than ICloud? One that does not reduce the file when copied and would work with OSX/LR...
Not sure it is iCloud that is doing the reduction. I think it is the Photos app that does that when uploading to a cloud base service. But it might only be the combination of both Photos and Apple's iCloud system.
Sorry I don't use any Cloud based storage system. I tried MS OneDrive but it lacks features so I dumped it in favor of a 64GB USB stick. That and network storage on my home network and multiple external drives connected to my main desktop computer.
Let me state this again.
If you are using LR for 99% of your image editing and management needs you need to Stop using the POS Apple Photos app. All you are doing is confusing both of the image apps you are using and yourself.