As is analysis of legacy systems

I’ve been tasked with performing an ‘as is’ analysis of a monolithic legacy system in my organisation. I’ve been conducting interviews with the technical team responsible for developing the system along with other techniques (such as but not limited to, observing people using the system, surveying, meetings etc), and I’ve been creating data flow diagrams to map out the system.

As I’ve never conducted an ‘as is’ analysis of a legacy system, what is the best way to model the system? I’ve been creating DFDs although I’m begging to question if this is the most appropriate approach.