Because you’re on 2x8, you’ll need new shifters, derailleurs, cranks, and chain, along with a rewrap for the bars and likely new cables and housing, Buying those aftermarket and asking a shop to install them will cost most of a brand new bike (that will likely have a nicer frame and wheels, too). I would recommend looking at a new bike with the components you want.

What’s the mileage on your Merida? Your shifters could legitimately be worn out. A friend of mine wore out his Sora shifters after about 10 years, likely due to a burr in his shifter cable causing extra drag. Also, did the shops change out your cables? That would help a lot after 7 years.