Without a crafting or gathering class to help you make money, I would suggest farming mobs for drops. Depending on how much things are selling for on the Market Board will determine what's most worth your time investment, but if I were going to go out to farm item drops to sell I would go after Deepeyes, Okeanis, Puks, Crawlers, Antelope, Aldgoat, Diremite, Dragons, Karakul Sheep, and I'm sure there are others I am forgetting, and you probably already farm most of these mobs, but these are the guys I would farm for drops to sell. Again, checking the market board before you head out to farm will definitely be to your advantage so you can invest your time farming mobs whose drops are going to yield you the most gil.

A slow way to make money is to buy items from vendors and put them up for sale at a profit on the Market Board. Most people don't take the time to actually seek out vendors for the items that they need and will instead go straight to the Market Board and over pay for something that they could have bought much much cheaper at a NPC sometimes even when that NPC is literally 3-5 steps from the Market Board itself. I hope this helps, though with you sitting at 2.5mil I don't know that I'm providing any info that you're not already aware of and utilizing, but again, I hope at least some of this is helpful to you!