First time doing something like this for SEO: built a JSON-based template for “alternative to X” pages. I know it looks dull, but I thought to plant the seed asap for any potential traffic, and improve later. What people expect to see in those pages?

Congrats! What people expect to see depends on the platform. Meaning i you want to build a tool for thought, then highlight it. If you want to build a bookmark management tool, it's only fair to show that. IMHO. That is, clarity about what the tool does — comes first.

I'd say even the way you decide to "highlight" the goal of the tools and the difference depends on what you're building exactly. F.ex, Cosmos's landing page clearly states their focus on visual curation. Arena is more "verbal" so to speak)

Clarity in what a tool does attracts specific people who decipher these features the way they expect a tool to solve their problem. If that makes sense)