I can’t see why such a device would exist in the form you describe - a pot is a pretty terrible absolute position feedback mechanism.
Remember, R/C servos are designed for model control, with a man-in-the-loop.
does not exist. it is like asking for a car that both floats and flies, but does not roll on the roads.
of course, you can make anything you like and it may work fine. I would offer that since you know what you want, that you should be able to make a circuit that drives your stepper the way you want.
simplest design would be a NANO or MINI, a stepper driver, power supply (don’t pretend you are going to use batteries)
and your pot.
connect your pot to the shaft of the stepper, run the pot into the A/I of the arduino for feedback, write your sketch to read the angle and then output to your driver to control. the only real addition to running a stepper in a more common way is the addition of the pot. but some software and you can integrate them.
Use a shaft coupler to connect two things that have shafts.
It doesn’t, because no one would buy one.
I would buy one. The applications would be any robotics project where someone wanted much more torque than servos provide and they wanted to take advantage of the simpler, direct-mount stepper shaft instead of messing around with external gear boxes to get the required extra torque. They make big beefy RC servos but they are $500. No need for that when the cost of RC servo internal electronics and the stepper itself is much cheaper. You have 5v powering the board and PWM signal, but 12v or more powering the stepper.
Why is open-loop bad? I want to tell the stepper to go to a specific angle, and not have to guess about where it was when I gave it that command, or calculate (guess because you don’t really know the real position for sure) the number of steps needed to get to that angle. I want to be able to interrupt it while it is in the middle of moving to tell it to immediately head to some new position. It needs to be able to be bumped or interfered with but still know exactly what position it is in. You can’t meet all of those requirements with open-loop control. I also don’t need continuous rotation. 180 degrees is fine.
If you are trying to optimize high-torque, direct-drive strength, & low-cost (< $150) while meeting the above requirements, what better option is there than the one I originally described? Show me a servo with > 500oz/in of torque, a metal case, and a 3/8 to 1/4 thick, > 2in long output shaft for under $150? Oh, they don’t exist? That’s how the initial question came up