Does anyone else think that over a period of years products like SAP provide diminishing returns for the efforts and money put in, over software built from scratch in house ?

Yes, software built form scratch can also be messy, but which is the lesser evil?

How about a 'filter' that can 'wash' the air with clean water. To me it appears to me the best possible solution to clean air in a confined space. To put it simply, air can be bubbled through the clean water, to scrub it of both particulate and chemical contaminants. No doubt the apparatus will lend itself to several technical challenges like noise and the need for periodic changes of the water.

I believe such air purifiers already exist.

They do exist.

The main problem of air washers is that it is not enough to change the water.

Fungus and spiders love the wet environment. Algae start growing if there is sunlight using just the dust particles on air as food. Also very bad bacteria like salmonella and others could grow if the water gets warm.

See: https://venta-usa.com/product-category/airwashers/

They do an excellent job of humidifying the treated space, which may or may not be ideal.

dredmorbius, do you own one? At what frequency do you normally change the water?

No, I don't. Ran across them discussing filtering options recently.

They do. They're literally called "air washers," and you can buy them on Amazon.