iCloud Photo Library will need a local Photos Library somewhere on your Mac, either in your Pictures folder or on a locally mounted external drive. To make the version of your library you copied out of Dropbox your iCloud Photo Library, double click the copied version to open it in Photos, then open the "Photos > Preferences > General" and click "Use as System Photo Library", if this option is not grayed out. Then open "Photos > Preferences > iCloud" and enable iCloud Photos Library.
If so, how do I get each iMac to only sync with the iCloud version... and each other?
Keep the library out of Dropbox. Your Macs and your mobile devices will sync with the iCloud version of your library, if iCloud Photo Library is enabled. iCloud will store the full version of your library and sync this library with the local libraries on your devices, so the Photos Libraries on all devices will be identical.
It is only 45Gb though, whereas it was 186Gb before..
Has it been 186Gb before you started the iCloud Sync, or before you moved it back from dropbox? Or before you started the Repair? It is hard to tell, if the syncing with Dropbox damaged the library so bad, that photos have been lost, or if the syncing with iCloud removed originals.
iCloud Photo Library can reduce the size of the local library , when you use "Optimize Storage". That is intended.
Do you have a backup of the library from before you moved it to Dropbox?