Are there any window AC units that don't hang outside of the window (building requirement)? Or is a dual-hose portable AC unit my best option for an energy efficient AC unit?

If they don't hang outside the window they'd have to hang inside the window, and would need a more complicated ventilation system to take air from outside, heat it up and vent it back outside. At that point you'd have a window mounted two hose AC anyway.

So yes, your next best option is going to be a two hose portable AC. One hose takes air from the outside to cool the condenser, one hose to throw that hot air outside.

Single hose works too, but they're less efficient because they take cold inside air, cool the condenser and vents it outside, which waste some of the air it just cooled for that and it creates negative air pressure inside which will bring hot air from the outside to replace it from any cracks and holes in the house.

If you have a balcony or patio area (assuming you are in an apartment), you could opt for a split A/C. These have most of the A/C "guts" in a box that sits outside and a couple of hoses that hook up to the heat exchanger inside. The outside box chills water, pumping it through the inside heat exchanger. They're more efficient than window A/C units, and get around rental agreements prohibiting window units.

If the ‘window’ part isn’t a requirement. A mini-split fits the bill. They are efficient, aren’t a blight, and can be sized to service a room or so.

You may be able to find a slim-profile window AC that sits within the window casing. They’re not very powerful, so plan on one in each room.