Combo Trail/Rain Pants

Showers Pass makes zip-off pants for biking. $150 retail. No idea on the weight. https://www.showerspass.com/products/mens-club-convertible-2-pant Nice idea, but…. I could see wearing full side zip rain pants with silkweight long johns for cool-cold weather shoulder season hiking, especially in the PNW, but they would get hot and sticky in anything 50F or higher and miserable in direct sun. Light softshell-ish pants is about as good as it gets I think. But once they soak through, you have sticky boat anchors wrapped around your legs. On one trip I was wearing soft shells and kept going in heavy rain because I was close to the trailhead. The DWR was good for a while, but once it soaked through, it was naaaaasty. A poncho and tall gaiters can make an alternative to rain pants. A poncho allows chaps too. There is the rail kilt thing too. No free lunch I'm afraid.

With all the features you mention I expect that they'd weigh more than regular pants and light rain pants combined – and not do either job as well, and not offer as much warmth when worn together for warmth. Plus I imagine you'd have one heck of a time sealing such a beast with all the pockets and so on. And you'd be replacing them much sooner than your rain pants since they'd get so much more wear. A better approach in my experience is rain pants and shorts and long johns.