Redirects seem straightforward enough.
Like I said just grab a list of:
- the important URLs (clicks, traffic, link, old sitemap based)
- the new migrated URLs on Shopify
Then create a redirect map 1-to-1. Quick look in a few tools (Ahrefs, search dorks) tell me there will only be a few hundred URLs. 2k max at a guess. Not that big of a migration project.
Create a redirect map, upload to Shopify, done.
^ samples tested work fine.
You don’t need to worry about the n-th degree of parameterized URL variations that were never getting any clicks or crawls. If you can create some bulk rules in Excel sheet great, but I wouldn’t worry about them from an SEO pov.
The only (minor) problem I did see was the old site has URLs under /products.html&&url=xyz - which is technically a ‘live’ page (/products is a reserved URL path in Shopify). This might be the one you mentioned? Although I only saw 18 URLs with any visibility under that pattern, no detectable traffic so shouldn’t be that big of a deal to skip, would suggest sanity check against GSC data.