There's something missing from your review: a discussion of relative SSD drive speeds. Apple's internal SSD is blazingly fast, easily 4x faster than when reading and writing from even a good M.2 SSD in the Satechi. That speed difference will likely not be noticeable for most people who just want more storage space, unless they will be editing very large video files or doing similarly disk intensive tasks.
However, the internal SSD speed is important for Apple's claim that most people are just fine with 8GB RAM. When a computer runs out of RAM, the slowdown is caused by swapping to the SSD. The swapping to Apple's internal SSD is generally fast enough as to not cause a noticeable slowdown for most users... even when browsing with lots of tabs open.
Once again, the caveat is that Apple's claim is true unless the user runs memory, disk and CPU intensive applications like full VM desktops, or high resolution video or photo editing software. (The trifecta of intensivity is where such slowdowns really start to be noticeable.) But those users are not what Apple means by "most people," and should not be buying the base model.