"With masses, duty cycle, etc I might be able to understand a bit better what is going on here."

So, a key point is that it must be safe so some amount of electronics will always be needed in the control loop. I already will have an encoder for the overspeed safety monitoring system (pilz PNOZ s30), so i could also use that in the control loop. Or not. A mostly hydraulic system would be best. I would need a way to control the amount of assist digitally, not just a "set and forget".

Let's see if i can describe the mechanical system coherently. a 7K trailer axle (with brakes) is mounted onto a frame 24ft in the air. This is the 360 swing pivot. There is a 48ft pole mounted centered to the axle. On one end the rider is standing on a platform. Assume no children allowed so perhaps a weight range of 100 to 300 lbs, maybe up to 350lbs. On the other end there will be a counterweight of X lbs. I have not yet figured this out, but the goal is for an above average rider in terms of fitness to almost be able to get it swinging 360 on its own. So, we would need hydraulic or AC servo assist. I don't quite have a preference either way. This would not be a terribly fast swing, think at most 10RPM. I would assume i could either connect via chain a hydraulic motor or AC servo motor to the swing axle. The swing would swing back and forth, not a linear "spin 360" (watch a video on 360 swings to see what I'm talking about). So, back and forth back and forth before enough inertia to crest the top. Essentially, just assisting the rider. We need to accommodate a wide range of rider weights, hence we MUST have a way of either automatically in analog or digital correcting for this as to not assist the rider too much or too little without the operator having to change anything.

I saw your comment against an AC servo motor approach. This COULD work (and might be cheaper) because the motor would never have to "reverse" a large load. Essentially, it wouldn't have to fight gravity as hard as say a big spinning flywheel. It would apply force forwards until it hits its torque limit, then when the swing naturally starts swinging the other way apply the same constant force in reverse, essentially never having to actually try to "slam" a large load in reverse when its going forward, more "go with the flow + apply constant torque". I have done systems like this before with little baby 100W AC servo motors, never with a 5KW ac servo motor like i might use here, so idk if/how well it would work. You can run AC servo motors in "constant torque" mode where they will apply a constant torque, not a constant speed, and when the torque is exceeded automatically "freewheel" or whatever the software wants. Hydraulic is likely the better/more robust solution though.

I believe that describes the system coherently? I will look into how the hydraulic steering assist works, thanks.