Hey there,
first up: I am not sure, if I understand you 100% correct and also I am quite new to Reaper as well.
If you'd really like to just have some simple volume automation accross multiple tracks, I would either:
1. Create a group and automate the groups volume:
2. ... or create a volume automation for every track by hand, create an envelope item with
Alt+Left-Click (I think it's the default shortcut), make the envelope and copy it with
Ctrl+Shift+Alt+Left-Drag (I think this is the default setting for pooled copy):
Is this what you wanted to do? Hope this helps!