Here's something I saved from the Reaper forum. It uses MeldaProduction's MAGC (in their free bundle). I want to try this in SONAR, both with MAGC and AutoGainPro.
Level-matching Reaper FX chain using MAGC I've put a JS routing plug (JS: IX/PhaseAdjustingRouter) as first insert into the FX-chain (for just sending the "dry" signal onto 1 more pair for further down the chain; like: 1/2 normal signal to be processed, and the same signal goes into 3/4, but is not heard and not processed, just used as Sidechain for Melda's MAGC at the end of the chain).
Your actual FX chain you work on.
Melda's MAGC is last in the chain, which is set to listen to the sidechain (coming from the first plug-in in the chain (pre-FX-chain-signal, on 3/4 (or 5/6 (see below))), and then this sets the level for the actual audio which comes in on channels 1/2. MAGC then sets the level of the main signal on 1/2 to the same level of the sidechain coming in on 3/4.
So, your FX-chain level always follows the level of the dry track, or the level whatever was before the actual FX chain you work on. Consistent loudness while working on your FX chain.
I've also tried TB-Audio's AB Level Matching Plugs but they were too slow for me. I find that MAGC reacts much more natural and quicker to level changes and is less fiddly overall.
In the attached .RAR. there is the JS Plug (goes like you said to /.../AppData/Roaming/Reaper/Effects/-here-). Here at my place, it was bundled with Reaper (in the FX browser it's called "JS: Channel Router w/Polarity" not like the file itself. Maybe it's already on your machine too anyway.
The FX-chain can be loaded through the FX-chain in Reaper directly. But copying it to your other Reaper FX-chains is not a bad idea in general. The FX-chains are by default located at /.../AppData/Roaming/Reaper/FXChains/)
Please note, that I've set the sidechain-input for MAGC to 5/6 and not 3/4. So, your track-channel-count will change to 6 channels on that track. That's because generally I use 3/4 for the compressor sidechains. So, I wanted to leave those inputs/track-channels free, for later use in my case. This way I don't have to worry if any other sidechain-input from another track will mess up my volume-matching sidechain-signal. You can change this of course, but then also change it in the Melda MAGC I/O pins, so that MAGC knows where to take the SC-Signal from.
In between those 2 plugs, you load your normal FX and work away.