You can have one library syncing with iCloud Photos. This library will be synced across all your devices and you can see the photos on your Mac, the iPhone, the iPad.
But as Winston Churchill explained, you cannot simply put a Photos Library on iCloud Drive (or any synced volume like DropBox or Google Drive). The syncing would rapidly destroy the library. You need to use iCloud Photos to sync your photos with iCloud.
When I started to use iCloud Photos I created a new library with only my favourite photos and the photos I am currently working with. All other libraries are archived on the external volumes and backed up on other volumes. This is working rather well. The iCloud Photos Library is small enough to fit onto the device with the smaller storage and to keep the list of albums small. It is easy to work with it even on the iPad to iPhone.