If I were in your shoes, I'd start with Standard Shopping campaigns, not Performance Max, and set my bids manually. That’s probably the opposite of what Google is telling you to do, but it gives you crucial control at the start.
Your goal in week one isn’t immediate profit, it's gathering data. By using Standard Shopping, you get access to the search terms report, which shows you exactly what people are typing before they see your products. PMax hides this. For a top-of-funnel campaign, this information is gold. You'll see all sorts of broad, exploratory searches. Your first job is to aggressively add Negative Keywords to block the irrelevant ones. Dan Meszaros's point about preventing wasted ad spend is critical here; you need to be ruthless about cutting off searches that will never convert.
After a few weeks, you'll have a much clearer picture of what's working. This is when you start optimizing. In month one, your focus should shift to placement. Destaney Wishon talks about this constantly on Serious Sellers Podcast in the context of Amazon, and the logic is identical for Google Shopping. Being at the top of the search results is incredibly valuable for conversion rate. I'd look at my campaign data and start using bid adjustments to push my most promising products into those top spots for the right queries. Jordi Beltran and Carlos Sastre on Ecommerce Braintrust talked about