Good question. This doesn’t just apply to manga but to other types of media as well (like anime). I mostly consume AV media like anime and film however so maybe take this comment with a grain of salt.

what even is the purpose of a review? Are reviews recommendations or are they intended for people who have already wachted/read?

The content of your review will depend on who your reader will be. You could have either one of the audiences, or both to some degree. A review as a form of recommendation means that you have to indicate what kind of “taste” you think the work appeals to. For example, I could say Bakemonogatari appeals to people who are interested in avant-garde cinematography. It helps to compare it to other works so that the reader has a point of reference.

If it’s intended for people who have watched or read it, then your intention is to invoke discourse surrounding the work. For example, I can say that I dislike DARLING in the FRANXX for its inability to fully challenge gender norms of the mecha genre and explain why using detailed excerpts from the work. This sparks discourse and helps readers make judgements about the work. They may or may not agree but that’s the point. Your job now is to collectively (and subjectively) define what new standards of media should be.

Second, what level of spoilers is considered acceptable, if its something greatly enjoyable and unexpected but it happens on the first 10 chapters, should that be mentioned?

The level of spoilers will depend on who the review is for. Is it for people who have not yet seen or read it? In that case, it’s better to be ambiguous or just hide content in spoiler tags.

If your intent is for discourse (for people who have watched or read the work), then go crazy. Start your review with the ending if you want to. Say how much you dislike the open-ending or mention how thought-provoking the image presented in the third panel of a manga chapter was like. IMO, the more detailed it is, the better you can support your argument for the work.

Third, what are the areas that should be focused on? Are there any kind of common templates or is it pure freestyle?

This depends on the writer and it’s entirely subjective. The area of focus entirely depends on you and what makes you value a certain work. It could be characterization, plot, image, sound, timing, social relevance, etc. You could also choose to focus on the strength of the work. You aren’t limited to one area either. You can be as expansive as you’d like. Typically, most people focus on narrativity and image, and use things like sound and characterization to compliment their arguments.

Personally, I’m not a fan of the template reviews found in MAL where they rate a work’s animation, story, characters, music, etc individually, then give an overall rating but this is usually how a standard template for reviewing works anyway.

The Cynic Clinic’s review on Azumanga Daioh uses the same format (but in a good way). He begins by briefly explaining what the work is about, discussing a strong point of the work, in this case, characterization, and relate how the other elements support that strong point (i.e., how the lack of a romance plot makes the show more relatable, how the visual style emphasizes the characters, or how the music and sound reflects the quirkiness and fluffiness of the show). He finally concludes with a summary of his points. Through the review, he ties in all the elements of the work together to focus on one thing: characterization. This style is very effective for people who haven’t seen it yet and it’s insightful enough for those who have.

What I really like are essays and reviews that have a central theme or idea to it. They’re much closer to an “analysis” of sorts but I find that they’re much more valuable if your audience is people who have already seen or read the work (as we mentioned earlier). The Cynic Clinic released this video essay on Bocchi the Rock appreciating how its radical style challenges the common notion of what adaptions should be like.

Try to experiment and see what works for you, though generally, I’d at least try to avoid writing like the popular MAL reviews. I’m not devaluing these reviews but oftentimes they lack research, critical thinking, and appreciation for what isn’t the norm.

Related, if someone has any guides on hand, or books, even if they are for other kinds of media, could you please tell me about them?

I really like Dunkey’s humerous Game Critics video. The Cynic Clinic has a lot of good videos like 4 Resources That Shaped The Way I Understand Anime, People Don’t Know How to Talk About Animation, and Nobody Cares How Many Anime You’ve Seen.

Sorry if this comment is lengthy. I know I mostly talk about anime (despite this being a manga community) but I hope it helps somehow.