Hello!
My main site for years has been http://SaveOnCannabis.com which is a WordPress based coupon directory for online cannabis items, affiliate links to outside sites where user purchases.
But over past years the site has lost lots of traffic and we need to try and recover the SEO.
Now we have been building a marketplace which is a Shopify build and is on the sub-domain of http://Shop.SaveOnCannabis.com but have been debating if it should be its own domain (like ShopSaveOnCannabis.com or SaveOnCannabisMarket.com or even SaveOnCannabis.Shop). Why we are debating this is heard bad to mix WordPress and Shopify sites on same domain, also since the main domain is already in bad graces of Google wondering if better to start that store on a fresh domain and also one that is all Commerce based (aka everything from own blog posts on Shopify site can be more purchased targeted content too).
But also why I’m conflicted is SaveOnCannabis.com has been around for 7 years and did have a solid reputation with Google before the great smackdowns, also it is just clean to have the main branded domain for everything.
What’s your thoughts? Also bonus, what domain would you recommend for the marketplace if I do separate it to its own domain?

Sounds to me like you need to keep them separate. I’m not an SEO expert, but if you’ve got seven years of juice, why ruin that?