Hi Guys
Thank you very, very much for your help. I'm now down to 61% Memory. So far no more Issues (even with the one remaining String). 
J-M-L
To be honest: I don't fully understand your Code but it works standalone. I've implemented it into my Sketch but it's too big for the Nano (101%). But I definitely keep it to study it closer and maybe could use it for future projects. Thank you for that !