I kind of wonder how viable it'd be to make a product that consists of:
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A strong smartphone case, with structurally-strong "dock" connector at the bottom.
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A "dock" -- maybe in a couple different sizes -- that links to this, has a pass-through USB-C hub, and adds stuff like a headphones jack. Maybe stick an integrated powerbank into any free space.
Smartphones don't have standard dimensions at all, resulting in a zillion cases out there, but having the case with a standard "dock" attachment as a separate part would mean that you don't have to build a million variations on the dock.
There are existing "put the smartphone in a dock" products, but they're aimed at putting the phone on a desk, using it like a laptop. I dunno if there's something comparable for just holding it. I haven't seen anything like that.
From a purely-electrical standpoint, USB-C permits for a lot of devices to be added. But physically, on a smartphone, that means carrying other boxes. A "dock" that just extends the height of the phone would avoid that.
If the only thing you want is a headphones jack on a smartphone, I'd probably just get a small USB-C-to-1/8"-TRS adapter and leave it attached to the headphones; they can be pretty small.