If I want a bit of cream and a bit of skim milk and a sugar free
flavoring, what do I ask for?
The easy part.
sugar free flavoring = "Zuckerfreies (Geschmacks-)aroma", e.g. "Zuckerfreies Vanille-Aroma" for vanilla.
skim milk = "entrahmte Milch" or "Magermilch" (0.1 - 0.5% fat) but normally you mean "leichte Milch" or "Milch mit 1.5% Fettanteil"?
cream = "Sahne" or do you mean condensed milk = "Kondensmilch"?
Info: there is really no meaning to consume such fat-reduced milk until you continue eating cheese, yoghurt (instead of skyr) and meat.
The complete opposite of your coffee is a Cortado.
Edited after question of OP how to order a hot coffee with hot skim milk (0.1 - 0.5 % fat).
"Einen Kaffee mit heißer Magermilch, bitte." = "A coffee with hot skim milk please."
"Einen Kaffee mit heißer Magermilch und zuckerfreiem Vanille-Aroma, bitte." = "A coffee with hot skim milk and sugar-free vanilla flavor please."
Tip: DeepL translator from Cologne start-up come with the same translation.