Serenity very dangerous as it is. Any small transport very dangerous in really. Its just doesnt intended to blow cruisers, but it can just run from it. Trader can also just dock without being PvP dead in full sense.
For heavy defence with alot of cargo there exists battletransports. Well, here i agree that choise not so big, and only bison/stegodon is true BT, shire is something worthful and all rest is trash, working shitty in convoys.
Of course its intended that military/policy/bounty hunters will defend you. Also if you want hunt/blast scum instead of trading, you can play those IDs and use appropriate hardware, like battlecruiser.