You need to determine if it’s the hydraulic steering assembly or the motor that’s stiff. Disconnect motor from hydraulic steering, swing motor by hand, should move pretty easy. If not that’s the half that needs investigated, bushes can seize on the motor. If motor moves freely could be a problem at helm.
As already said, disconnect the steering from the engine & see if the engine is nice & easy to push from side to side.
Pointless doing anything else until you've established that.
Engine swivels can seize & sometimes takes a lot of work to free them off. Guess how I know.....