In my post I stated that guiding worked with the indi_synscanlegacy_telescope driver. I can not reproduce this with a real camera. Likely I did something wrong during my first test. Maybe the guiding interface of the indi_simulator_ccd driver was used.
Asking the Sky-Watcher support regarding guiding interface of the SynScan App was not helpful.
Next days I will have a deeper look in the code of the indi_synscan_telescope driver.