According to my lbs it's worn out.
It's been adjusted and tuned today but advised to change the lot as it still doesn't shift smoothly and only a matter of time until the chain snaps.(again)
This derailleur hasn't been hit.
SRAM 12 speed is very sensitive to how it's set up, but, there are many happy users out there.
There's a few things that could be happening here: I hope that you're not offended by any of my questions, I'm trying to help and you haven't given all of the info that we need to know!
- your LBS might not be serving you well, either by not setting the bike up properly, and/or now by wanting to sell you stuff that you may not need.
- your LBS might be correct, and you may have damaged the drivetrain via lack of adjustment and maintenance
As background, your cassette should last for multiple chains if you replace the chain before the chain gets to 0.5 on the wear scale. If you don't have a tool to check this, any decent bike shop will stock them
If you have used a chain that's past 0.5 on the wear indicator, then you may have worn out your chainring and cassette by doing this (as background, older 9 and 10 speed stuff can be used to 0.75 on the wear indicator)
Have you been checking the wear on your chain? How worn is it now? How many miles have you done? How old is the bike?
Also, I don't think that it's a good idea to "live with it" when the bike isn't shifting well, especially when that starts a week after buying the bike...when a simple adjustment may have put you back into the sweet spot.
Summing up my thinking: if you've neglected the adjustment and maintenance, then this might be the root cause of all of your issues...and the same could happen to a replacement drivetrain.
Moving on, and assuming that the chain hasn't been used past it's service life...
...I'd go back to basics, and ensure that the following has been done by the LBS...before spending money on a cassette or chainring. Doing these things will ensure that the bike has been adjusted properly:
Install new housing (Shimano SP41)
Install a new cable
Install a new chain (if it's close to 0.5 on the wear tool)
Check the alignment of the der hanger (THIS IS CRITICAL)
Check the setting of the B-screw, this MUST be done with you sitting on the bike so that's its sagged
Check the adjustment of the upper and lower limit screws on the rear der.
Adust the cable tension so that the shifting works in all gears
IFF at this point, your shifting is still not great, then the cassette and chainring probably needs to be replaced. Your LBS wants to do it all at once because it's easiest and most profitable for them
If you don't want to worry/think about your bike, or don't have the time or interest in maintaining it, or if you're not that knowledgeable, then you've hit the nail on the head by going back to 10 speed. It's less sensitive. I have an old 10speed XTR shifter with XT cassette...and the shifting is as smooth as butter, and not that fussed by e.g. cable adjustment.