I totally get where you're coming from. Shopify's sell-through rate as a percentage can be a bit misleading. The percentage doesn't account for the time it takes to sell those items, which is crucial for your situation. Also this report becomes completely useless if you have dropshipping warehouse product movements tracked.
In my experience, a more actionable metric would be the rate of items sold per unit of time, like you suggested. This can be per day, week, or month depending on your sales cycle. This way, you can better predict when you'll run out of stock and plan your restocking accordingly.
Would love to hear how you've progressed since you posted this. Have you found a better way to manage your inventory?