Hello
Unfortunately Germany has nothing within this period of jet aviation we are in until we move onto the next stage of aircraft. The F-4F was something retained by Germany until later operational aircraft like the Eurofighter took over.
As such, there currently is no 13.7 possible in the German inventory to add.
Sadly this is a factor of some nations which others have already experienced previously (Japan, Britain, France, Italy, Sweden) where there were periods of time when they did not have a current top fighter that was a direct counterpart to what other nations had, leading to a gap period where they do not have a top BR fighter jet in that current moment.
Where it’s possible we will always find gap fillers and solutions, however the German gap is something that will be rectified in a future major update as all others were previously.