Tips for optimizing the UI/UX in a Shopify Plus store during a redesign?

I’m in the process of redesigning my eCommerce site for outdoor gear (think hiking boots/backpacks/tents) that’s been running on Shopify for a few years, but it’s starting to feel clunky with slow load times (about 5-6 seconds on mobile) and a high bounce rate (like 45-50%), especially on product pages. The site has about 200 products, custom themes with some outdated code, and we’re seeing drops in conversions because the navigation isn’t intuitive: users complain that the search bar doesn’t filter properly and that the checkout process has too many steps. I want to focus on the modern UI/UX to make it more immersive, such as adding better zoom capabilities on images, streamlined menus, and maybe some AR previews for gear if possible.

To address this, I’m working with Tempe’s Fyresite. They handle the custom development side, including migrating some elements to Shopify Plus for better scalability, optimizing the backend with AWS for faster deployments, and redesigning the interfaces to prioritize user journeys (for example, faster add-to-cart buttons and personalized recommendations). They’ve had a discovery phase where we identified pain points, and now we’re in collaboration mode customizing wireframes for things like responsive layouts that work flawlessly on desktops/phones/tablets.

What metrics should I track before and after the redesign, such as GTMetrix scores or Core Web Vitals? How do you integrate performance improvements (CSS/JS minification, lazy image loading) without breaking custom apps? And any advice on A/B testing new designs before full launch?