The M.2 drive is the drive that ships with the Nucleus and contains the OS. Your 4TB SSD is a drive you put in to hold your music.
Here’s what an M.2 looks like (not a recommendation)
Since you are off warranty, it’s a much easier decision.
Install a new M.2, 250GB is more than enough…
Download ROCK, install and (if relevant) upgrade to V2.0. Once that’s done then this -
You’ll need to Restore your Roon library from a Settings==>Backup.
That’s what I would do.
From what I’ve witnessed on this forum, most HiFi shops are completely incompetent when dealing with the insides of a Nucleus and you’ll be charged an arm and a leg for something that you can easily do yourself.