If I understand correctly, you are suggesting to combine different sensor types in one thing.
IMHO this would jeopardize the concept of the binding. The binding provides one thing per message type (see https://www.mysensors.org/download/serial_api_15). the thing has one channel per supported sub type.
I also do not see the advantage of combining different sensor/message types in one thing as you need one item per channel anyhow.
=> this would not reduce the number of items needed.
Grouping the items in the site map - as you already did - is the best way here.
Does anybody see this differently?