I'm about to purchase my first MacBook with 8 TB SSD and 128 GB RAM. As a music producer, I wanted to know if importing all my sample data from my external hard drive to the MacBook drive was safe. I will write lots of data that would be transferred from the external drive to the MacBook to lower its lifespan (I'm transferring all of the data in one go, by the way). Should I just get an external hard drive?
The durability of a Macbook pro SSD is the same as any good quality internal SSD on any computer. No difference there. The only difference is that you can not replace the SSD yourself in a Macbook (or any modern Mac computer) if it fails. As with any good quality internal SSD, under normal circumstances, it will last for a long time (at least 5 years but probably longer). With exceptions of course.. you can always have a dud, but that goes for all parts in any computer.
But why do you worry about the durability of the internal SSD? As a music producer I assume you have a regular backup plan in place? So if the internal SSD fails you have your backups right?
Never rely on an internal system disk only, because your OS is also on it. What if something goes wrong with an OS update and your computer is bricked? What if your computer needs repair? Apple does not guarantee that your data will be kept after a repair.
To answer your last question: Yes (regardless of the quality of the internal SSD) you should always have at least one (but preferably more) external hard drive(s) for backups. Preferably also one outside your home (in case of burglary, fire etc.). And (depending on how much you produce) backup your projects regularly.