thank you for your comment and perspective. I will try to play around with the tags and implement your feedback
Best look how other people do it, that have a similar situation.
For starters: https://itch.io/games/steam-key
Maybe there is a pattern how their Itch presence differs from their Steam presence.
Also, https://itch.io/games/tag-3d-platformer But I am not sure if your game counts. I think your game is https://en.wikipedia.org/wiki/2.5D but I do not know which tags are common for that.