Thank you for your reply Gina @flareAI
I can confirm that I can see the subdomains appearing in my existing sitemap, so my existing sitemap has updated. Google is indexing the new subdomain pages. I’m still not sure why the advice in Shopify support seems to suggest submitting different sitemaps for each subdomain. Maybe it’s something to do with going down the subfolders route? Anyway my primary domain sitemap seems to be working correctly as you say.