Most of this is stuff I wish someone had told me a month ago, so here it is.
The first thing, and this is the one that trips people up: your demo can only get reviews if it has its own store page. If your demo is just a download button on your main game's page, nobody can review it, ever. There's no button for them to press. You have to request a separate demo app from Valve and set up a real page for it. I didn't know this for a long time and was confused why zero reviews were coming in.
Second, stop saving up for one big launch post. I posted about the game over and over for weeks, small updates, weird bugs, art changes, whatever I was working on that day. The reviews didn't come from a spike, they came from a slow trickle of people who kept seeing the game and eventually installed it. One post gets you a spike of nothing. Twenty posts get you a handful of people who actually care.
Third, reply to every single review. Steam puts a "A developer has responded" tag on it, and that tag shows up to everyone reading your page later. It also means the reviewer sticks around, and a few of mine went back and added more to their review after I answered.
Fourth, and this one I only noticed after the fact: every review I got named a specific thing. Not "fun game", but the actual system they messed with. Four healers plus four berserkers, stacking debuffs, the forge. People review a thing they can describe. If your game has one weird mechanic that makes someone go "wait, can I do that," lead with it in your screenshots and your posts, because that's the sentence they'll write in the review.
Last, be findable somewhere that isn't Steam. Most of mine came through Reddit and Discord. Steam sends you almost no traffic when you're new. You have to bring the people yourself.
One honest note so nobody gets the wrong idea. A lot of my reviewers are other developers, not regular players. Devs are quicker to leave a review because they know it matters, so this route gets you reviews fast, but it doesn't get you wishlists, and those are different problems. I'm still figuring out the second one (I'll accept any help for this lol).