It's the flexibility that does it. Python is a multi-classed ship that barely gets outclassed by the more dedicated ships it competes with. It's probably too good imo, from what I gather the Anaconda is kinda similar in that regard. Currently flying my Python around doing some hauling missions and enjoying the views in VR. As you mention it's got 256T of capacity whilst still having ok shields, decent jump range and enough fitting for weaponry. Despite downgrading the power plant, power distributor and engines it's still manoeuvrable enough to throw around, land on planets and fight off lower end stuff. Don't underestimate that benefit of being medium sized too, it's useful being able to land at Outposts! For comparison my Python has a 4A shields, shield boosters in the utility slots to give me some bugger, 256T cargo, 3 large multicannons, 2 medium pulse lasers and a jump range ranging from ~25LY to ~39LY depending on load. Way beyond 10ly
Not flown the Type-7, can see it would be useful on a cost basis over the Python, but once you've got the latter I don't think it's worth looking back! I think the cost is the only thing that really justifies the Type-7. Once your at the Python it's arguably not a huge stretch extra to look at moving to the Type-9 and have ~3x the cargo.
Re: Interdiction. From what I understand the larger the ships the harder it is to evade interdiction. The stats suggest the Type-7 is a brick compared to the Type-6 so would struggle to evade a lot more. The latter is somewhat similar to the Python.