@Russ The problem was not the paths. It turned out that smartthings-cli wanted a fresh login by root.
I tried logging in to GUI with root, but was unable to initiate the login prcess that way.
--token <uuid> after each command did the trick!
(<uuid> being the token from https://account.smartthings.com/tokens)
Cron and other services that start at boot are still unable to run the commands.
Now I get "Error: did not get any data from stdin".
I bypassed the problem by running a python script manually. This script stays open and executes the smartthings commands as needed. The downside is that if the power goes out and RasPi reboots the script does not run automatically.