The structure of what I had in mind was going through a laundry list of aspects of code (functionality, style, structure, functional decomposition, algorithmic efficiency, test coverage, use of libraries, blah blah blah), and making specific positive comments about the things that are already good. Personally I don't glean much value from that i.e., the compliments aren't something I'm concerned with and the feedback isn't something I can act on anyway.
OTOH, by going through that laundry list, it shows that you actually performed a full review. You can optimize that (for example, by having a checklist where you mark off all areas with either constructive criticisms or a checkmark.
Could you give an example of what you're imagining a longwinded nice version would be like? Because this seems mostly fine, just a bit clipped.
All I'd really expect to improve it is something like: "This is mostly fine. It just needs tests for negative numbers, empty strings and null." Adding just a few words validates their work, and more practically, it also clarifies that the criticisms are exhaustive.