PowerBI Query - transform, assign and move Data into other fitting cells

Hello Power BI Board,

its a little hard to describe my problem, but i have to following challenge infront of me:

I cleaned and transformed a Dataset into the following Dataset:

(the last column is not actually part of the initial data,i just needed to create it to piviot my data (maybe its somehow useful)).

(the empty cells show as (null) in the Query editor)

Now i need to do 2 things:

1. delete rows which dont have a Source orTarget System and therefore have no connection and cant be assigned to something

    (like the 3 rows in Activity "ccc" which only has Target Systems)

2. connect somehow the other systems through filling out the blanks correctly.

   (somehow fill in the blanks of the rows of activity "ddd" with "TSystem1" etc)

So essentially transform it into this:

I am struggling a lot with this task so if anybody has any idea how to do this in the Query editor (what steps) or through new columns with DAX etc. i would be very thankful!

Strathos