If at all possible, the best fix is to get more memory in your computer.
Most of us can't do that, so firstly, I'd suggest using the output settings instead of the fast render. You get more control over your render.
And if you can't render to a video file, then try rendering to images. PNG is a good format to use. Then if OpenToonz crashes while rendering, you'll have something rendered. Then you can change the frame range to start on the next frame. And then keep doing this until your render is complete. Then you can assemble the images into a video using a video editor, using ffmpeg direct from the command line or create a new scene in OpenToonz, add these images and try rendering that to an MP4.