It is inevitable that you will see some rankings drop, no matter how well you check all the "migration best practice" boxes. I've never seen a migration done that resulted in no, or even positive changes right after launch. However, you can greatly reduce the time that your rankings fluctuate and stabilize again by taking all the proper steps. There are simply to many to list, but the main issues are typically around URL changes. Ideally you'l want to make sure there is a 301 redirect in place for every page that has a URL change. Also, make sure to submit a new sitemap via Google Search Console the second you launch the new site.

The thing to keep in mind is that any time Google sees big changes, it will "step back" a bit from the site to reindex and reanalyze everything. Whether this is a platform migration, an extended server outage, or a domain change, it almost always results in a temporary decrease in ranking. Whether or not (and how soon) you come back stronger will depend on how well you handled the migration and how much better the new site is from an SEO perspective. Shopify is a decent platform and like any of the major systems, can be optimized to rank quite well in most industries. I'll leave you with this decent resource - https://moz.com/blog/web-site-migration-guide-tips-for-seos (I'm not affiliated with Moz in any way).