In my humble opinion, yes a new starter motor is needed.
I had exactly the same on my 2012 GSA when I still owned it. Farted about with new and extra HD cables to the starter, service kits for it, but a new starter cured it. Does the starter get hot quickly? If so it’s drawing loads of current and something is amiss.
Your local auto-electrian place might be able to rebuild the one you have. I got an aftermarket Arrow branded one. £175 IIRC. Make sure it’s the right mine though: these bike have been fitted with Valeo(?) and Bosch starters and are a slightly different shape although I believe interchangeable but fiddly to fit the wrong one.
I git one for about £160 if i recall
i'll have a dig through my paperwork and see if i can find it
It was a reman unit
solved my poor starting instantly -
BUT a great way to test the theory 
) ,No better . Removed started , stripped it to parts , reassembled with a new brush box - - all good ! ( so far )

