hard cranking after warm
Charging voltage won't tell you in this case. You need to measure STARTER voltage WHILE CRANKING. Check connections and grounding, inspect cables. Then "rig" your meter with clip leads right to the starter post and to the engine block. Absolute minimum in my book "while cranking" is 10V, and should be more
If you have that and it drags, pull the coil wire. Might be kicking back?
If that has no effect, it's either the starter or you have an engine problem, tight bearings, spun bearing, or something wrong in piston/ ring fit
Your "new?" (rebuilt) starter does not mean "good." It could be defective