My favorite is: "Our buy now button should be orange, that converts best" without testing it, verifying the results, and just pointing at some blogposts.

can you recommend good groups?

A lot of Shopify groups.

Search up anything Shopify or ecommerce mastermind.

Be aware that a lot of them are set up as groups that simply try to sell you stuff.

The best ecommerce folks are basically the best marketers.

Thanks!