Although I also use iCloud, all my photos come together on my Synology using Synology Photos. They are automatically uploaded and stored in Folders by month. I browse and edit them with Nitro on Mac. Seamless workflow. All I had to do is add that top level folder to the sidebar in Nitro.
I’d recommend to not have too many images in a folder because loading via network is slower. Typically I have around 100-500 photos per monthly folder which works just fine.