M.2s are dirt cheap. For Nucleus OS, you probably don’t need anything bigger than 128GB, but get a 256GB to be sure.
I would get one that has some heat mitigation built in ( I suspect the M.2 deaths on Nucleus have to do with heat) or add heat sinks to whatever you get.
If you take it to a dealer to fix under warranty, they’re just going to send it somewhere and who knows how long that will take. They’ll probably replace the M.2 with the same sh&tty brand as the original.
OTOH, opening the case (to replace an M.2) will probably void any warranty you might have. Dunno.
If you do replace the M.2 you then need to download and install ROCK, which is easy enough. Once ROCK is installed contact support and they will update the OS to the Nucleus OS. See this post.
You might want to wait and let @support have its say about this and other things.
Then again, if it’s still under warranty, it won’t cost you a penny. That said, I’ve long replaced my M.2 ssd with a Samsung 980, even when the stock one was still working properly. You’ll have to install ROCK on your own, and ask support here to convert the installation to Roon OS. This last part is important as it contains thermal management optimisations (Nucleus is passively cooled).
Why? So they can replace the M.2 with the same crappy brand as the original and then have the M.2 fail again when the device is off warranty. Not to mention the wait time to get the problem resolved. Sorry, I must disagree.
The only problem I see is if a user replacing the M.2 invalidates the rest of the Nucleus warranty.
This sounds like a motherboard issue, not an SSD issue, unfortunately. You can try to replace the SSD, but I suspect it won’t help if the issue is elsewhere. I would involve your dealer and we can start an RMA process with them or directly with you, if you prefer.