2018 YZ450F starting issue (not starter clutch)

I have 40 hours on the bike, never had an issue with it. Didn’t ride much this year, maybe 4 hours at most and last time was early august. last week i tried to start it but the starter was spinning the engine very slowly, as if the battery were dead. I tried charging it, tried another battery, same issue. I disassembled the starter and there was a bit of dirt in it. Cleaned it all up and put it back in, same issue. Everything looked like new inside. Bearing is smooth, all magnets are in their place, brushes are good.

I can bump start the bike easily and it runs like new.

I have researched a lot and came up with nothing, as if no one ever had that issue. Does anyone have an idea of what it could be or what to check, before I bite the bullet and order a new starter without being sure it’s the source of the problem?