How can I upgrade MacBook Pro disk space?
You could probably go with your 1st option from an Apple Service provider or DIY, but Apple doesn't upgrade storage on any shipping Mac. The last one to offer that was the Mac Pro which had no-tool, user swappable hard drive bays.

Your 2nd option will tie your laptop to the network providing the NAS for full operation. You would want an iSCSI initiator for the fastest storage, but NAS would be just fine for large files and folders you access infrequently.

I propose a third option: external storage. 

I taped a 1TB SSD to the lid of my MBP. Not that attractive but very effective. Thunderbolt is the fastest DAS (direct attached storage vs network attached storage) but USB 3 is no slouch either. 

Be sure to tape the connector and cables as well to prevent jiggle disconnects.

I didn't care about the looks. Consider this a proof of concept (tape is enduring nearly one year :-) So there is enough room for improvement. But I'm very happy with this solution. I travel daily with this setup with all my music and stuff. 



Pictures of opened lid