We rebuild the page closest to your sale and run it against the one you have now, on your own live traffic. The scoreboard is revenue per visitor, not a dashboard metric.
Usually the product page, because that is where the money is decided. We rebuild it end to end rather than tweaking a button colour.
Old page and new page live at the same time on your traffic, so seasonality and ad spend hit both equally. The result decides.
Two or three tests a month after the rebuild, each one run to a clear answer. Fewer opinions, more tested changes.
If the rebuild does not lift revenue per visitor, you keep your money.