I cannot figure out how can I check a status of synchronization of a given folder with a given client(s) (preferably, how much butes/percents left till “Up-to-date”), using syncthing-cli on one of the nodes (“master” node in my setup)
afaik syncthing-cli isn’t maintained anymore and is not fully compatible with the current version of Syncthing.
You can use the REST API to get the data. Specifically /rest/db/status to get your local folder status and /rest/db/completion to get the completion percent for a specific folder and device.