there's about thirty miscellaneous spelling/minor plot errors
There are two schools of thought on typos. One is they're fixed after everything else, as other tasks can add potentially typo-laden material, so it's the author's responsibility to sort that. The other is that they should also fix typos before it goes to a beta reader, because it's not right to waste their acumen on such objective details. Either way, they're not what you should focus on. If they're rare, you can mention the occasional example you couldn't help but notice; if not, you can mention they're frequent.
Your "/" suggests you're comparing some plot errors to spelling errors based on their size and importance. Are they even plot errors? "Why did character X do Y?" questions often have a plausible answer you can easily think of; if you're only four pages in, they can be a hint of something that will be revealed later. Either way, they may be as unimportant to mention in detail as the typos to which you compared them.
plus a page of feedback on the bottom about larger plot, personality, description, and word repetition issues, as well as a bit of overall feedback and advice for moving forward.
That's what beta reading should be for. Your feedback shouldn't be a quarter of the text's length, but many of the issues you notice now will recur and not require re-mentioning. As a rule, focus on patterns. Word repetition issues, if they need to be dealt with at all, can be mentioned concisely and then you move on; you needn't even give examples if you think the author should develop their own eye to them. Furthermore, Ben Blatt's Nabokov's Favourite Word is Mauve notes a distinction (and also concedes an overlap) between two types of words an author uses more than is average in the language, "cinnamon" words (which are still acceptably rare) and "nod" words (which are distractingly common).
How harsh is too harsh when reviewing/critiquing? By too harsh I mean it offends or hurts them in some way and makes them discouraged when writing in the future.
If you worry about that, it's because you're a good beta reader. All you need to do is make sure you also mention what you liked, even if it's just things other authors frequently mishandle that this one didn't. (Positive feedback still won't be a majority of what you write; if the author's smart, they'll understand why!) But don't separate positive feedback into its own section; if you were already working approximately chronologically, continue to do that.