How can I test the checkout process before publishing a Shopify theme to ensure it doesn't break?

The biggest fear when changing themes for a DTC independent site is that 'the page looks good, but checkout conversions drop.' Once, we launched a new version of a PDP; the design and speed were great, but the conversion rate from mobile add-to-cart to checkout dropped significantly. After troubleshooting, we found it wasn't the payment gateway, but a conflict between a bundle app's script and…