My recommendation for combat rank: mix it up. Do the Thargoid Scouts, but also take some pirate massacre missions and hit up a few RES, grabbing mats and cash while improving rank.Hmm. Thargoid hunting. Need more Guardian stuff. Time for another visit to that region then... I might tool the Anaconda up for that but it'll be a while.
In the meatime, here's a gratuitous screenshot I like: love what B type stars do for the look and feel of planets.
If your going for some more Guardian stuff, shoot plenty of Sentinels, they count too.