Trade. Your position was the exact point I realised I had to change what I was doing to get into the bigger league ships. I went initially for an Asp - with mostly cargo and A grade FSD I was about to get around 100t about 25ly or so. That got me up to about 30 mil in assets.
Around this time the requirements on the Clipper were relaxed down to Serf and I found one in Li Yong-Rui's territory. This gave me a 15% discount and I was able to get a decent enough trade spec. 240t of Imperial Slaves, buy for about 13k/t and then sell them somewhere that buys them for about 16.5k/t. Each run nets around 750k. Rinse and repeat, upgrade your ship or save it up for something even bigger. I upgraded the Clipper because I really enjoyed flying it, lovely ship.