Non-Mining Nodes don't validate anything,
they only RELAY blocks according to their consensus rules, they don't validate anything at all.
Which is why non-mining nodes don't really matter and their is no security advantage to having more non-mining nodes.
If nodes just blindly relay, then why wouldn't they relay an invalid block? You said it yourself, they relay blocks which conform to consensus. Which means non-mining nodes are enforcing consensus rules.
You are free to believe the misguided notion that non-mining nodes don't matter, but you won't find much support for that kind of foolishness here.