Advice with shifter issues and chain lube.

I have a Specilized Rockhopper Sport 29 2021/2. And I recently was having issues with shifting on the rear derailer, only to find the wire that does the shifting has frayed through half of its strands. Tried fixing this myself, but only made it worse. I took it to my local bike shop and they said it was likely that the shifter and derailleur were incompatible. The micro shift 9 speed and shimano derailleur came stock with the bike. What would cause this? For example they were able to tune the bike after replacing the cable and housing, but the bike still will jump on gear 7, which leaves me a big gap in my range. 2×9 gear set. Lastly I was told I should get chain oil to help with shifting. I wasn't going to pay $25 for an 6 oz bottle. So I ordered lube on Amazon. Is the one I ordered good, or should I go with a different brand in the future?