When I ran a store we used to have support contact the customer. The verification process changed a few times, but usually just from making contact you can get a feel for if it’s a real person or not. If unsure, we would ask them to take a picture with their ID. Could be invasive to some people, so you could start lighter (explain your automated fraud system flagged the order, ask them about the details, like why they are placing the order from a different country, or whatever the details are).
I did an analysis and shared my experience here: How to Deal with Fraudulent Orders and Minimize Chargebacks on Shopify
If you’re letting 0 high risk orders through, you’re probably cancelling legit orders. You might consider starting small, like letting x% of high risk orders through, based on your criteria of them being the least risky, start there and increase it until you start getting chargebacks, then dial it back to find the sweet spot.
This depends on the order value of course. Like gambling, if you’re not comfortable risking the money (product) then just cancel it for peace of mind.