@mh6824213 Most of the good methods are already in this thread so I will just answer your last question directly, since thats the interesting one. Here are the things that showed up in audits that nobody would ever catch by clicking around their own store.
Shipping rate gaps. This is the worst one because its completely silent. If a country or a postcode range has no shipping rate configured, the customer gets to checkout, sees no shipping available, and leaves. No error, no support email, nothing in your reports except a slightly worse number. Way to find it: open Analytics and look at sessions by country, then find any country sending you decent traffic with zero orders. Then go to Settings, Shipping and delivery and check that country is actually covered by a zone. People set up a domestic zone, add rest of world later, and leave holes in the middle.
Default variant out of stock. If your product page defaults to the first variant and that one is sold out, someone landing from an ad sees a dead button before they touch anything. Feels fine to you because you always click the size you have. Check your handful of top traffic products and see which variant loads first.
Orders that were never really orders. Your conversion reports stop counting at order placed. Cancellations, refunds and undeliverable addresses all sit on the other side of that line. So conversion can go up while money goes down. Worth pulling your cancel and refund rate for the same period you are optimising, otherwise you can celebrate a change that actually made things worse.
One thing worth saying about checkout, since a few replies mention checkout friction. Unless you are on Plus you cannot edit checkout. So most checkout CRO advice is not actionable for you. What you can control is Settings, Checkout, things like whether accounts are required, address autocomplete, and how tipping shows. And shipping rates, which is where most of the real friction lives anyway. Aim there instead of trying to redesign a page Shopify wont let you touch.
Last thing, and this is the unpopular bit. If you are doing less than roughly a thousand orders a month, A/B testing will mostly waste your time. You will not reach a result you can trust before the season changes underneath you. At that size the wins come from finding things that are outright broken, like the three above, not from testing button colours. Testing becomes worth it later.