I feel your frustration because I have been looking for a sweet-spot jiving translation of this idea from German into English several times already. Me being a native German speaker. This saying "Erst die Pflicht, dann die Kür" is taken from performance art sports, like figure skating and perhaps dancing and gymnastics. "Kür" has no other extant meaning in German really than to skillfully move about as in dancing, showing off one's technical skills in the most artfully intriguing way. In that context "Pflicht" is more specific than just "duty", it means your required moves and jumps you have to check off to show your skills in a technical run-down, where your freedom of interpretation and expression is limited, and only once you pass that, may you show of your double head-over spin backwards jump.
This is the meaning of the saying.
The only way to really translate it into English is to use at least one of the two contrasting words to really signify the performance art sports competition rules. If that is the word "freestyle" then so be it, unfortunately, however, freestyle has a wider meaning than German "Kür", as freestyle means you can move how you want (e.g., freestyle swimming race) but it doesn't connote so much the artful expression, the dancing that is implied by "Kür".
So there may just not be a translation that hits the sweet spot, which is probably the reason why this saying doesn't exist in English language, despite the sports reference readily available for English speakers. It just doesn't jive as it does in German, because "Kür" is a very unique word.