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).

I think Stoney is on the right track here, but the only thing that's missing is that this question cannot be answered correctly without actually knowing the industry and current leaders. There are different industries and topics that Google has different tolerances for. If authority websites on your topic commonly have/receive lots of backlinks from bloggers, you have more room to play with in this area. Also, social media is not a requirement for SEO. We've seen plenty of sites outranking competitors with no social presence at all. There is no one formula for what SEO should look like across all industries. The only place I'd say you're definitely looking at raising a red flag is with the anchor text. In your example you said you'd want thousands of links back using the same keyword. Chances are, no matter what your topic or industry, that's going to look very out of the ordinary. A natural looking, balanced link portfolio is really key these days in offsite optimization. You'll get all sorts of people these days that tell you to "build content people love" and SEO just happens. That's not the case, especially in an industry with any competition at all. SEO is a deliberate, calculated marketing strategy. If you feel that you're in an industry that's even remotely competitive, then I'd recommend seeking out an SEO agency with a solid reputation. If you're bootstrapping a business and looking to do whatever you can on your own, then I'd start by educating yourself on SEO from reliable resources such as: https://moz.com/learn/seo. (I'm not affiliated with Moz in any way). Best of luck!