Hi Luke,
I love Shopify's simplicity as a platform but, in my experience, Wordpress gives you far more control over your destiny in terms of technical SEO.
Wordpress is installed on your servers and you can pretty much do what you want with it. You own it and you don't need to adhere to any terms and conditions or worry about your content / site being removed.
Shopify, on the other hand, does have some limitations because they don't really let you anywhere need the engine of the site, especially near their servers. When you're renting on somebody else's land then there is always potential risk.
However, SEO success is not based entirely on your platform but the quality of the content on your pages and the popularity of that content. I've seen some slow, god-awful sites rank at the top of google for some very competitive terms.
Many people think SEO is just a few meta tags and keywords on some pages and that's job done, but it's far from that.
If you create a real SEO strategy that's fit for purpose then you will get the results you want, but you need to put a lot of effort in. This isn't a 'set it and forget it' type of thing.
Keywords, content, technical, speed, usability, user-journey - they all matter. You need to know what you want our of your efforts rather than just blindly adding blog pages and sticking some links into them.
Also, it's important that you get the right customer to your site. SEO is often a
'drive for show, putt for dough' business. Visitors mean f-all if they do nothing for you when they get there.
Focus on the niche customers / searches and become an authority for your audience. Write awesome, digestible content. Solve their problems, be useful and don't do the hard-sell.
If you do this then your popularity will rise and people will start to share your content, link to you and do your marketing for you.
Anyhow, here's a good breakdown of Wordpress vs Shopify SEO (not my site) -
https://www.stylefactoryproductions.com/blog/shopify-vs-wordpress
I hope that helps.
Matt