Any kind of delay would require a change to the device firmware… and, to my knowledge, homebrewing Zigbee firmware is still pretty niche.
That seems reasonable to me, especially if the sensor has limited access to routers on the mesh.
Are there any locations in your house that have line of sight even when the truck is there? Installing additional routers in those areas might help.
Is it the same van or driver every day? Maybe you could plant a BLE beacon on the roof of the van… or surreptitiously install BLE tire pressure caps… or track the driver’s phone. 