Three things from the other side of this — my game is exactly the kind you are trying to surface 🎮
IGDB sourcing will miss the long tail you are aiming at. IGDB’s coverage of itch’s smaller releases is thin, and “flies under the radar” and “not in IGDB” are close to the same set. If the index is IGDB-first, the platform structurally excludes the games it exists to find. Dev upload helps, but it is opt-in — and the devs who never market are exactly the ones who will not opt in either.
A recommender built on likes and collections cannot solve cold start. A game with no signal is invisible by construction, which is the precise problem you opened with. So whatever you do for a game with zero likes and zero collections is the product. If that path is content-based — genre, tags, mechanics, description text — say so loudly, because it is the one thing that separates you from every “people who liked X also liked Y” system that already exists.
Be careful trusting self-reported mood and feel tags. Devs tag badly, and not dishonestly — they tag what they hoped they made. Ours went out carrying “pixel art” and “puzzle-platformer” on a 3D Three.js arena game, and it sat that way until an audit caught it. Cutting the wrong tags and adding the right ones did more for us than anything else we changed. If mood is dev-supplied, plan to validate it. If it is derived from the build or the text, that is a real moat and worth leading with.
What does a game with no ratings on day one look like in your system? That is the interesting half, and it is the half most discovery platforms quietly skip.