Shopify takes care of a fair amount for you. Clean URLs, decent site structure, mobile that works, SSL, a sitemap you never have to think about. That is genuinely more than most WordPress stores start with.
Then it gives you a few very good ways to undo all of it.
Manufacturer descriptions
You imported the product feed and every description is the text the manufacturer wrote. So is every competitor selling the same item.
Now there are two hundred pages on the internet with identical copy and Google has to pick one to show. It is not going to be the newest small store.
You do not need to rewrite all four hundred products. Rewrite your top twenty. Say who it is for, what it is actually like, what people ask you about it. That is content nobody else has.
Empty collection pages
This is the biggest miss on most stores. Your collection pages are your best shot at ranking for the searches that actually convert, and they typically contain a title and a grid of products and nothing else.
Put a few real paragraphs at the top. What is in this collection, who buys it, how to choose between the options. Not keyword mush. Actual guidance, the kind you would give somebody standing in front of the shelf.
Deleting URLs
You discontinue a product and delete it. That URL had links pointing at it and rankings attached to it, and now it is a dead end.
Redirect it. To the replacement product if there is one, or to the collection it lived in. Shopify has redirects built in and it takes ten seconds.
Same applies to renaming things. Changing a handle changes the URL. Always leave a redirect behind you.
Images with no alt text
Product images are a real traffic source through image search and yours are probably all named IMG underscore four digits with no alt text.
Describe the item plainly. Not a keyword pile. What is in the picture.
What to skip
Most of the SEO apps. They scan your store and give you a checklist, and the checklist is roughly what I just wrote, and you now pay for it monthly.
Do the four things above yourself over a couple of afternoons. That is most of the available upside for a store your size.