It’s easy to smooth the dynamics of midi velocity in Reaper to create a more compressed even sound.
First, open up the item needing compression into the piano roll.
Select all the notes. Then right click on an empty part of the grid (not on a note) and in the dropdown menue select: EVENT PROPERTIES and change the type to NOTE
Then in the Velocity box you can use *(multiply) or / (divide) to reduce the overall dynamic of the velocities. For example using *.25 or /4 will give you a 4:1 compression ratio. Hit apply and the velocities will be squeezed.
Then while everything is selected, grab the velocities and bring them back up to around 75% or wherever suits your piece.
Need more compression – just repeat the process. I usually find this process plus a little bit of manual tweaking is a really quick way to get a more even sound.