High-paying bulk cargo runs -- been there, done that, got the T-shirt. Polymers is how I filled up the missing last 5% to Trader Elite. Took 4 runs. ^^
The thing is, there are snags with every high paying loop.
Military Fabrics: there is only ONE port that pays top dollar, and it's planetary. The nearest producer is a bit away, so an unengineered heavy freighter will need 2 jumps. And of course the planetary landing and lift-off eats a lot of time. There's nothing useful to ship back, but at least that means you can hustle back in one jump. I did that loop a couple of times but ultimately decided it's too much trouble.
Survival Equipment: also gets high offers from many buyers... but ALWAYS at planetary sites, so I didn't investigate further.
Polymers: Many of the high-paying stations are outposts, so forget them for bulk runs. The Large stations always seem to have their own catch:
Either there is no supplier nearby (minimum 2 jumps), or one involved station is many 1000 Ls out. Or both. And again, there's nothing useful to ship back.
That said, a good Polymer loop is to Carabineri or whatever it's called... Caberani it is. I managed to drop 4 loads in a little over an hour, for some 25 Mil profit in a Type 9.
Then finally the Elite bar was filled up, and I sold that slug the minute I got the message in my inbox. xD