I suppose it is possible for it to be a solenoid issue. Because when activated the Solenoid pushes a plate against 2 contact points inside of the Solenoid and it is possible for those contact points or the plate to be not making good contact and not allowing enough amperage or voltage to turn the starter properly.
The attached diagram is a Bosch Starter. The green circles are the high amperage contact areas. On most Starter there is a circular plate instead of that wedge shaped on shown.
Picture of my worn Starter Brushes in post #36 and in #37 the armature.
Starter rebuild issue...
Anyone have good source for starter rebuild parts