If you are losing a lot of water via the pump, then you must have quite a big leak which would be obvious as there would be a puddle under the offside of the engine. There are 2 possible areas of major leaks. 1) Some radiators have plastic sides, these are prone to cracking due to age & heat. 2)There are 2 hoses, flow & return, that connect to the heater matrix these have plastic T piece connectors that are prone to cracking due to age and heat, they can be replaced with brass barbed 19mm equal Ts. See link for the pump part number, plus other parts, then use as a cross-reference on eBay https://www.amayama.com/en/genuine-catalogs/epc/toyota-japan/alphard/ATH10W/43189/engine/1601