If grinding maximum credits is your main goal, you'll likely get bored with ED fairly soon. My advice is to try to find things to do that are both enjoyable and profitable. Also, vary things a bit and don't get stuck in a rut.
The other alternative to straight up trade hauling is focusing on the "VIP Passenger Transport" missions. You will need to buy yourself a dedicated passenger ship like the Orca to get the most out of these, but it is totally worth it!
In case Synthetic Frost didn't already know...
These specific transport missions are always a simple A to B run of under 100Ly that pay anywhere between 3 Million and 12 Million credits! If you setup an Orca with 3, 4 or 5 first class/Luxury cabins and engineer your FSD drive to a Grade 5 range, you can stack 3-5 of these missions into one run, and if you cherry pick the highest payouts, you can come away with upwards of 40-50 MILLION CR for less than 30 minutes of flight time! (An Orca with a Grade 5 FSD upgrade can do 100Ly in 2-3 jumps depending on the build/weight of cargo)
The various drops tend to all be within 12-30Ly of each other, so once you have made the trip out to the first system, the others are all one jump away. You will often end up with two or three going to the same station as well, in which case you can wind up completing the entire run in 10-15 minutes.
These missions are what I relied on to build up my free cash amount to 4.8 BILLION CR as of right now.

A great compliment to PowerPlay as well, since the 50 Million per week salary for a Rank 5 doesn't always cover all of your fortification/preparation expenses.
EDIT: To get the max payouts, you need to be Allied with the mission givers, but this is also worth the effort for obvious reasons.
