If you would like this to work you can only use one automation. For it to work well best to group your motion sensors as suggested above but you must use ‘binary sensor group’ as that is what you have. If you are unsure how to do this I have a step by step guide for you. You then use that group as your trigger not the motion sensors entity ID. You can possibly get away not grouping them as you say your motion sensor clears in seconds and it is a big room but I would still recommend grouping them because more than 1 person may be in the room.
You can also use my 
Sensor Light blueprint and use night lights to toggle between 60 & 40 % brightness. To get the blueprint just use the import blue button at the top and follow the instructions. Once done go into automatons / blueprints tab at the top and click on sensor light one.
Hope this helps you
Blacky 