Alternatively you can freeze the tracks, which i think is way more simpler (and cleaner) for committing FX.
The settings for rendered media (glueing, freezing, subproject, etc) is in Project settings under Advanced tab IIRC
But..REAPER will use the media with that format for rendering the master output, so it’s not advisable to lower the quality of the rendered media. However, you have options to choose the resample quality for Playback vs Render—also could be found in Project settings
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