This is a common question, and as you’ve noticed, the official features only support one dim level per Automation/routine.
So you have two choices. You can make one automation for each device, giving it its own dim level, and use the same trigger for all of them. It’s annoying, but it works.
Or you can go to core which will allow you to create just one automation with all of your different devices. The set up for that is much more complex, but there are lots of community members who will be glad to help you.