Probably the simples approach for you is to join:
https://forums.cockos.com/showthread.php?t=183143
OSCII-bot is also an option. You will need:
* pattern file for OSC surface plug-in in REAPER which interpret input and sends feedback for parameters you need. Default pattern is almost what will work, but some default settings in it can be not what you expect (especially "...FOLLOW...")
* OSCII-bot which converts signals from/to MIDI/OSC form:
a) buttons are strait forward (MIDI<->OSC values : 0<->0, 127<->1.)
b) faders are linear, PB MIDI messages (14bit value), so MIDI<->OSC : 0...0x3fff - 0..1.. Well, almost. From what I know, at least some Behringer faders are 7bit in resolution. So check the max value they send, can be 0x3f00. But I think simpler to switch faders into CC in this case (at least for X-Touch Compact that is the case).
b.I) Mackie expects fader feedback when you manually move the fader, fortunately that is the simplest and default behavior
b.II) faders also send touch, as note on/off
c) encoders should be in relative move, note that they send not what they expect to receive for ring indication. Mackie mode has special Mackie encoding. In simple mode, that is probably direct linear (MIDI) value (I do not remember for sure)
* MIDI loopback software with proper routing.
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So, you see... Better just use Mackie surface or join mentioned thread, things are way more complicated otherwise
PS. an alternative solution is in my plans, but I am no longer sure it comes this year...