Start in Shopify Analytics, not a heatmap app. Behaviour / conversion funnel, split by device. If mobile add-to-cart is fine and checkout dies, stop touching the homepage. If the leak is the landing page, then look at the HTML.
The thing we keep finding on AU stores: the first-fold hero is loading=lazy, or it is a 2000px+ file with no width discipline, or an autoplay video sitting in the first paint. You can see that in View Source in thirty seconds. No PageSpeed score required.
Then Clarity if you want recordings. Then theme. Do not invert that order.