> 3. Germany spends quite a lot on foreign aid, which 'could' be understood as a risk preventing defense spending.
I was skeptical, so I looked it up. You are correct—Germany does spend a large percentage of its GNI on foreign aid [0]. However, as you put it 'could.' Ultimately you are comparing apples and oranges. Defense spending is different from foreign aid spending, although they may be related. So, this is not a strong argument IMHO.
> 1. The 2% target is a non-binding guideline.
Not fulfilling a non-binding obligation is also not a strong argument. It's a non-binding obligation for me to to wash my hands at home after touching raw chicken. It would still be irresponsible of me not to.
> 2. Germany has (for some time) a plan in place to increase the defense budget to meet the 2% target. It will increase on a year-by-year basis.
IMHO this is your strongest argument, but the fact that the level is not there yet after so many years of NATO, leaves the policy rightly open to criticism.
0: https://en.wikipedia.org/wiki/List_of_development_aid_countr...