Make some friends, form a wing and have them escort you. Also what are you doing super-cruising around the system you just died in only 10 minutes ago and expecting a different result? Live and learn I suppose, but maybe if some place seems too dangerous you should avoid it? In PP, control systems and especially headquarter systems can be very dangerous as enemy players will actively hunt in those systems. There's little the AI can do to help you in this situation besides be an annoyance to whomever interdicted you. Had you survived long enough, the AI would've arrived on the scene but it does take some time. Things don't teleport to you instantly.
Transport ships are big targets; not only because you're slow and mostly defenseless but when you're in PP you are obviously contributing to your Power. Enemy Powers will try to stop you. If this bothers you, then consider trading in something more defensible like an Asp or Python and learn how to avoid interdictions. Sometimes submitting is the best bet as it will allow your FSD to recharge quickly, however if they can mass-lock you this may not be the best choice. Chaff, silent running when you have no shields, and heat sinks are all your friend when trying to evade.
As to if you should play in open I say yes if you enjoy a challenge. Solo to me is very monotonous and I never feel threatened or, more importantly, entertained. I might as well go vacuum or do dishes if I were to play solo; at least then something unexpected may happen like a hairball clog my vacuum or a dish slip out of my hand. And yes, I trade in open.
Mostly agree with all of your statement with one amendment - I've found in open flying a transport - armed or unarmed - no one gives a rat's behind about whether you are in a PP or not.
Carrying cargo, no cargo, unaligned to any PP, with a PP, just sight seeing - doesn't matter. Flying a transport in open for any reason at all is simply a target, period.
Sometimes I do it just to see if I can get past players camping a route, sometimes go back to solo or mobius to rebuild the cargo I just lost, but I swap back and forth between transport and non-transport, in exact same system, same time period roughly, and it is night and day.
Virtually unmolested great deal of time flying some non-trade ship, swarmed like flies in a transport.
So if you play in open with a transport, a) protect against crippling loss by having insurance plus enough cash reserve to rebuy at least 2-3x your entire cargo seed haul (e.g. the money you need to make money), and b) shout raspberries at all the "pvp'ers" who make so much noise about wanting open to be "pvp" yet consistently ignore the challenging targets like my combat python, but go for the baby seal scenarios
This does not exclude btw that some players of course are just being smart - why go after a low value non-transport when there are bigger fish to fry? that is totally true and valid - but also true is given volume and tone of so many "pvp'ers" who keep claiming they're in open for the "pvp" - it's beyond natural probabilities odds that so consistently, there is so little griefer pvp in my combat ship, and so much target on back spidey sense in my transport.
e.g. if there was even a decent proportion of players who were true challenge pvp players, I'd be getting jumped a lot more in combat ship - hence my inference that it is all talk and not much substance to the hot air