missed the 12:01am pt slot on product hunt and went live at 9am - how much does that actually cost

The board runs on a 24 hour cycle that resets just after midnight Pacific, and everything added later in the day competes for the leftovers of that same window, so yes, you lost hours you can't get back. It is not fatal.

Send your list now. Those 300 people are the highest-intent traffic you will get all day, and holding them back to protect a ranking is backwards - the ranking exists to get you traffic and you already have traffic. I'd rather finish sixth with 90 signups than second with 20.

Ran two. Day one traffic on both was real but shallow: a few thousand visits, signup rate about a third of what my normal traffic converts at, and nearly all of it gone within 72 hours. The durable part was three or four conversations with people who actually had the problem.

Measure the day in conversations, not placement. Placement mostly buys you a badge and some backlinks.

Disagree with relaunching later. You get one clean shot at the 'new' framing and burning it to chase a better slot is a bad trade. Also, the whole point of scheduling is picking the date and time in advance - if you got it wrong once there's no guarantee the second attempt goes better, and now you've spent your list's patience twice.

Launched in May and made the same timezone mistake in the other direction - set it to midnight my time, which was mid-afternoon Pacific. Two things helped. I posted a comment on my own launch describing the specific problem in one paragraph with no marketing in it, and I replied to every single comment within about ten minutes for the entire day. Ended up with a decent comment count and finished better than my position at hour three suggested.

Also: make the first image a real screenshot, not a gradient with your logo on it. People decide in about a second.

Ask for comments, not upvotes. Comments are what make anyone scroll far enough to see a mid-page product, and asking people to upvote is the fastest way to get your day looked at by a human.