I am allowed park on a Sunday between 6–17 without using a parking disc. – Correct, Sunday is not included on the additional panel, which means that you should just refer to the parking sign, which states that Sundays do not have any time restrictions.
I may never park longer than 2 hours at this location. – Wrong, you can park for longer periods during times that are not restricted by the additional panel.
I am not allowed on a Saturday between 18–21 without using a parking disc. – Wrong, you may park since the additional panel only applies until 15:00 on Saturdays.
With a parking disc, I am always allowed to park at this location for at most 2 hours. If I do not have a parking disc, I have to pay a fee during the hours indicated on the additional panel. – Wrong, you never need to pay a fee. You are not even able to pay a fee!
Parking (E19) ”The sign indicates that parking is allowed in a parking area [...] parking is permitted, during weekdays except for weekdays before Sundays and public holidays, for a maximum of 24 hours at a time, unless other information is provided on an additional panel.”