If you have already taken it to two
mechanics and they both say "it's normal" I would think you are OK unless they are both worthless as bike mechanics. However, if you are still unsure, I have seen the inner hub axle (the one inside the wheel, not the quick release) break on Shimano hubs which caused rough spinning and cassette wobble. You can easily check this while re-lubing your bearings. If you want to attempt the job yourself, there's a nice video
I found that shows the whole thing step by step. You''ll still need a chain whip and the proper cassette tool as well as some decent grease and a few pedal wrenches that are the right size.
Looking for inspiration, everyday.
There's nothing a man can't "fix" with the right size hammer.