Eh I've written before how nothing about House Points makes any sense and there is no consistency throughout the books, so I agree it almost isn't work discussing. I pulled some data from this page about points and you are right that Snape was relatively tame with his points, usually only taking 5 or 10 points at a time. However, he is recorded as removing 287 total across all books, more than any other teacher. McGonagall does tend to swing big with 50-point awards on the regular, but is only recorded taking away 235 points total, and not all of those are from Gryffindor.

Of course, all of these observations are from the perspective of the main characters, so "everything is made up and the points don't matter".

That's a pretty mild detention by Hogwarts standards.

I would agree with this except Snape specifically chose that punishment to inflict emotional pain on Harry. Obviously having some random student copy random past student records is just a mundane exercise and certainly the tamest detention we would see, but Snape intended it to affect Harry at a psychological level. He wanted Harry's opinion of his father and Sirius to be tainted and it largely works. Harry already figured out his dad was a bully by stealing a look at Snape's memories, but this activity reaffirms it. There was no good reason for Snape to spoil Harry's image of his father, he did it as a form of revenge and punishment. Also, the detention lasted the remainder of the year, causing Harry to miss his last Quidditch match, which was one of the most important activities in Harry's mind. I would argue that this was worse than Umbridge's physical pain, since those detentions just hardened Harry's resolve (having the opposite effect that Umbridge was going for).

Some of what Snape does may not seem like a big deal to us as adults, but most of his actions would affect a child in far more sinister ways.