I'm using a Folding bike for its step thru benefits,

the recent, Oregon made, Bike Friday PakIT, is pretty light. (simple drive train)
and because of its folding and EZ disassembly, small wheels,

you could probably put it behind the seat in your truck.

70 on the coast

Ditto, look for a good folding bike.

I can't think of a single model rigid frame step through bike that weighs less than 30 lbs -- most are closer to 35-40 lbs, because they're designed with flat terrain common to The Netherlands or places like NYC where the weight isn't as much a factor.

Any rigid step through frame bike could be designed to be closer to 20 lbs, but there's no market for it in the US. I know several women who ride pretty seriously and none of them would be interested in anything that looked like a "woman's bike" or casual bike, even as a second bike, even if it suited them better for some rides.

Heck, I'd be interested in a lighter step through frame errand bike for my local grocery and errand rides. My errand bike weighs 35 lbs, closer to 40 with rack, panniers, cable lock, etc. And it's a compact frame with sloping top tube, not really as convenient as a step through when the rear rack is loaded down and stacked high. I need to roll the bike to a curb to hoist a leg over the back when I'm hauling a load of pet supplies, or to stabilize the wheels against the curb so I can kick across the top tube.

So far good advice from Fietsbob and Canklecat. Most people discount a folding bike but it can it check all your boxes. I know you listed weight and style but there is also price to consider. Bike Friday Pakit is a great bike but it starts at $1150 for a single speed and price just goes up. The question is where will you be willing to compromise? My money would be on the Gates belt drive four speed Pakit as that would fit your needs.