I think it's more important to learn the vast range of REAPER's functions and actions little by little than to customize it. Many people are so unaware of REAPER's capabilities that they end up buying plugins or installing scripts to do things that REAPER does out of the box.
And the reason many people find REAPER difficult isn't because of the operability or the GUI, but because they haven't learned how to use it.
By the way, today I learned an automation action.
SWS/BR: Set closest left side envelope point's value to mouse cursor (perform until shortcut released)
If you assign this action to a shortcut, you can drag the value of the envelope point closest to the left of the mouse cursor while holding down the shortcut, without having to click to select the tiny envelope point.
This action is very useful, but it's not very well known, is it?
There seem to be a lot of long-time REAPER users here, so I'd love to see some useful actions and how to use them.