Holding flights and hotel bookings like that is possible although any kind of online travel agent doing it at scale will not last very long. Travel is still pretty relationship focused when it comes to inventory and if an OTA tries to do things that create churn on that inventory then it will get flagged pretty quickly.
An aggregator or AI assistant like Muse might have better luck although the mechanism for those will require charging whatever credit card you give it. For flights, it should work just fine in the US. There is a mandatory fully-refundable 24 hour cancellation period on all flights booked more than 7 days in advance through US point-of-sale, or they have to honor a 24 hour hold without payment but not both. Hotels get a bit dicier since the policy there will be dependent on each hotel.
An aggregator or AI assistant like Muse might have better luck although the mechanism for those will require charging whatever credit card you give it. For flights, it should work just fine in the US. There is a mandatory fully-refundable 24 hour cancellation period on all flights booked more than 7 days in advance through US point-of-sale, or they have to honor a 24 hour hold without payment but not both. Hotels get a bit dicier since the policy there will be dependent on each hotel.