Seems like a lot of people make the mistake of thinking they can grind rank quickly at Ceos/Sothis AND make a lot of profit. If you are accepting payment in credits, how can you build REP? To build RANK you need to build FED REP.
First, if you are not yet ALLIED with all the local FEDERATION factions at the stations in Ceos and Sothis you are doing business with, then you have not even begun. You need to stack as many data delivery missions as you can for the FED factions and you must accept payment in REP, not credits. Eyes on the prize CMDR, you are not here to get rich, you are here to build RANK and to build RANK you must first build REP with the local FED factions. Once you are ALLIED with all of the local FED factions they will offer more missions and more lucrative missions.
The DBX is a good ship for this. A cargo rack or two will give you added flexibility in the missions you can accept. If you are unable to stack your 20 in data missions you can accept a cargo mission or two (from FED factions) to fill in the corners. I ran my missions between New Dawn in Ceos system and both Sothis Mining outpost and Newholm station in Sothis system but, I understand that new stations have been added. I started at New Dawn, accepting all data missions from FED factions going to either Newholm or Sothis Mining that paid in REP.
I then flew to Sothis Mining (fewer interdictions working outward to inward) and delivered all missions destined there, then accepted FED data missions going to Newholm or New Dawn. Flew to New Dawn and did the same, remembering to accept FED missions paying in REP, then flew back to New Dawn. Rinse and repeat, if I could not get my stack of 20 data missions I would accept a cargo mission or, accept data missions from Imperial (IMP) factions. May as well get a start on building IMP RANK, too.
This way I reached rear-admiral in a couple days of casual play. Once rear-admiral I started accepting missions paying in high-level materials and credits until I got bored, loaded up on Sothis Crystalline Gold at Newholm station, flew back to the bubble and sold that for a hefty profit, chased by pirates the whole way. o7