It is very important to note that both implementation methods are mutually exclusive one one site. The moment the Audience Library is enabled and AA data collection hosts are configured by Adobe, all Analytics data will start to flow to AAM server-side. So, if the client keeps both DIL and AppMeasurment module, the data will be sent twice to AAM.
So I wonder if there is a way to use the SSF integration way, but deploy DIL code just to use DFP destination, and not get the data duplicated, or if I should go directly with the DIL method from the begining.
There is nothing that we can do using DIL and not using SSF in AAM. So, DFP destinations are supported with SSF implementation as well. I need to get in touch with documentation team for the doc link you provided.
To configure DFP as destination, that can be set up as a s2s destination by AAM client care or your consultant. You need to provide them details like DFP account ID and type whether is it just DFP or DFP_BY_GOOGLE.
The SSF method still allows for cookie dropping by assigning segments to cookie destinations. Once the onsite code is updated you’ll see the AA request changes from an img request to a js request, inc the json response which use to be returned by the DIL demdex call. This response includes the information needed to drop the respective cookies on page for DFP to pick for targeting purposes.