If you read the discussion page you will notice that building=yes is very common, the other tags are disputed.

So for now it is fine to stick with the established scheme.

Routing is a completely different topic.

neither a building polygon nor a node is connected to the road network.

Usually the routing engine will direct you to the nearest node. There is a lot of room for optimizing the routing. For example if there is a paring space near your destination it could ask if you prefer to route to the parking space instead to the building.
Or in case a building entrance is is possible to route you accordingly in case the building is at a corner.

I suggest not to mix it with landuse.

In a country like Germany it is easy to distinguish different landuse areas. In a residential area it is often not allowed by law to run specific businesses. So you will for example not find a car repair in a resident area.

While in Thailand there are areas that are plain residential or plain industrial estate, there is no clear distinction in smaller villages. People who run a business often live in the same place.

And for routing the landuse is not used at all.

Stephan