YES! I managed to get it working :woohoo:

I found out that I don't even have to do anything on my Raspberry Pi. Just connect the cables to my mount and camera's and start Astroberry Server. It is even possible to start it in CLI mode (command line interface). No UI necessary.

Here is how I did it:
Locally I connect my joystick to my computer and in a command prompt I start an INDI server for my joystick using "indiserver -v indi_joystick".
Then I start KStars on my local PC and configure a remote setup in EKOS.
Mode: Remote, Host: Raspberry Pi IP address (pressing the Scan button will automatically fill that in), Port: 7624.
Check the INDI Web Manager checkbox.
Select the Mount, CCD, Guider and for Aux 1: joystick
Remote: "*@<local_pc_ip_addres>:7624" because the indi server for the joystick is running on the local pc.
And of course select the Primary and Guide scopes.
Then start the INDI Web Manager and create a new profile (e.g. called "Remote Setup")
For local drivers select the Mount, CCD and Guider (e.g. SynScan, Canon DSLR and ZWO CCD).
For remote drivers enter "indi_joystick@<local_pc_ip_addres>"
Then press the Start button in the INDI Web Manager.
Save the Ekos Profile and connect to it.
In the SynScan tab, subtab Options click on the Joystick Enable button.
You can assign the various joystick axis and buttons in the SynScan Joystick subtab.

That's it, with these settings you should be able to control you mount with a locally connected joystick.

Jurgen, thanks for the hints.

Kind regards,