Thanks again for replying, Jewel!
My site structure changed, yes, but I redirected all of the previously structured pages to the newly structured pages, like for like, ie:
(old structure) - thespacecollective.com/product/t-shirts
(new structure) - thespacecollective.com/t-shirts
This isn't the issue.
My issue is that the newly structured website wasn't built on a subdomain of thespacecollective.com, it was built on a completely different domain; thespacebazaar.com (and the latter website was, for some unknown reason, indexed by Google). My question was regarding thespacebazaar.com general redirect to thespacecollective.com.
I was worried that because Google indexed thespacebazaar.com (and subsequently all of its content) before it was transferred over to thespacecollective.com, would Google think thespacebazaar.com was the original author of the content and see thespacecollective.com as copying.
Also, thank you for looking at both websites and taking the time to see the differences.
The two sites are very similar (this was by design), and share around 50% of the same products. I am currently writing different descriptions and product titles for both sites so there are no content copying issues. So I don't think there would be any issues there (but again, thanks for checking).
As for performing an SEO audit...
I'm currently teaching myself SEO because everybody I have hired in the past has been terrible (and super expensive for the amount of work they have done). Essentially I am reading a lot, comparing my website to what I have read, and then making changes where necessary (sometimes asking for advise first). But I'll take a look at GTMetrix and SEOQuake, thank you!
I'll also take a look at your website, thank you!