@acer Thanks, i did that change from eval to subs, too and changed the latex/diff function in order to get \partial instead of the \rm d. Whereas i am not sure if it is a good idea to alter the latex functions directly, it gives me what i want. Still, any "cleaner" suggestions are appreciated! Thanks a lot.

@Carl Love Tanks for the procedure. In which way can it be risky to assume symbolic? I'm trying to get a more deep understanding, so I would greatly appreciate any explainations.

@Rouben Rostamian  Thanks, this works so far, but as my problem is larger then the example still looking for an elegant way to solve it. If the expression is larger, say

then simplify needs

to yield the expected answer

As my expression are growing even larger there is hopefully another way then setting all indets >0 for each simplification step. If I use assume(), i cannot isolate anymore for my variables..