Jist trying to brain storm here since I find this could be an issue I face in due time. I have not yet used Renoise to trigger external devices, but my setup is a little different anyways even if its mostly hardware, I use those to trigger synths or drum machines without going through Renoise via midi cables. But afaik if the render algo does not allow you to listen to the live changes but records them nonetheless, it’s a cool feature anyways.
Renoise is a little let’s say constrained when it comes to extensive multitracking audio recording but that is not really the intended design of a sampler/sequencer/tracker combo anyways,but says more about our expectations from it. So the best solution for this since you are relying on the render function is to use an external recorder and a simple mixer setup to distribute your outputs from the synth to both Renoise and the external recorder for both live use and use within Renoise of it matters to you this way. The simplest way is to just feed Renoise and the synth to their individual inputs on a basic mixer and record via a recorder to an SD card or to the hard-disk , which will still be real time.
Automation also might work if your synth supports CC changes and though I am not sure if Renoise supports full CC midi output it’s worth trying to see if those CC values can be automated for your purposes. Alternatively if your synth gives off CC changes for the knobs then just record them to a midi track or in Renoise and let it do its thing.
A Beringer MX 200 4 channel simple line mixer is one of the most basic you can find. Xenyx models are very good too. Yamaha makes some really awesome mixers. For portability Zoom R8/R16 have SD card multi track recording along with drum machine and guitar FX support and are totally worth their price. Then BOSS BR 80 series of battery operated multitracking studio pocket gear show enough promise though I am not totally sold on it for various reasons. If you have any recommendations please provide them here.
Maybe you could describe your setup so we can have a better picture of your situation.
PS. I like the profile avatar icon you have, cute, are you a female by any chance? Just asking!