Just to expand a bit on that.... a RAM upgrade is trivial and only takes a couple minutes. Apple intended that users upgrade RAM themselves, and they even have instructions on their website.
But Apple clearly did
not want users to upgrade the internal disks and designed the mini in a ridiculous way that makes it difficult to open, with fragile parts inside that are easily damaged. Plenty of threads in this forum where users messed up their mini by opening it. Not saying it can't be done, but there is a definite element of risk.
Regarding internal vs external SSD's, I have a 2012 quad Mini with the original internal Apple 256gb ssd. This is how it performs
But I now primarily boot that machine from a 1TB external Samsung T3 USB SSD. This is how it compares
So there is a definite speed advantage to an internal drive, although more with read than write speed. But I really don't notice the difference in everyday use. It is still very fast running from the external SSD. The only difference that stands out is that the internal drive boots in about 15 seconds vs 30 seconds from the external. That really doesn't matter to me. I use this machine exclusively for video/audio editing, so it gets a good workout.
Also consider that if you have the base Mini with the original Apple 500gb hard disk, any ssd will make a huge difference. That hard drive only has a read/write speed of about 100MB/sec.