How to write side-note reviews on assignments without sounding testy?
I can see you've given some thought to balancing the negative with some positive. That's good. Note, you don't need to remark on every single thing they did right.
If you have a rubric, a list of elements you look for in each lab report, you may wish to put a check mark in the margin wherever you see that a required element is present.
Sometimes a student doesn't manage to get much of anything right. In that case, it can be helpful to notice something positive about what they were trying to do, e.g. "Glad to see you caught this contradiction."
Try expressing things as questions, rather than statements. Examples:
Can you also check such-and-so? How does this fit in with etc.?
One of my professors would write in my proofs, instead of "Does not follow" or "Not relevant,"
How is this germane?
As a student, with certain graders, I was sometimes unsure whether a particular comment was positive or negative or just food for thought. So try to avoid those misunderstandings, e.g. "Food for thought: etc." or "Enrichment idea: etc." or "Conclusion is correct; be careful to make clear the connection between…"
Be tactful — meaning, your belief that they are trying hard, and can get better with your guidance, should come through. Example: "Watch out, need more digits here to avoid rounding error later on."
I suggest you arrange with the professor to visit the class briefly at or near the beginning of the semester, and maybe at the midpoint again, to introduce yourself and show a friendly face. Convey to them that your job is to give them lots of specific feedback to help them improve their work. This can help humanize things.
Let them think you're testy or overly picky. They will still take your feedback on board if they care about doing a better job next time--either for the sake of their own skills or simply to get a better grade. And yes, they'll complain about you, but that's no reason to worry. Thinking back to my student days, the professors and graders that my classmates and I complained about most, were often the ones we learned the most from.
Regarding tone, you're right that a neutral tone can come across as harsh in writing, but it's something people can get used to from you. In fact, once they do get used to it, this way of communicating is refreshingly efficient.
At most, you could ask the professor if he thinks the tone of your comments is okay, but I suspect you don't need to change anything.