You should not expect an optimization like this to converge in 4-5 steps. There is no oscillatory behavior seen in the log file, just 4 steps with large forces, but that's just the beginning of the optimization. It may take 15 or 30 iterations, hard to predict.
What I would change, to speed it up a bit hopefully, is to reduce the Si-C bond lengths from the beginning; the atoms have to move a long distance because those bonds are clearly too long, and that takes time. 9 to 15 k-points or something like that in the C direction should be fine., this is not a device where you need 100 or 200 (for very specific reasons). That would speed up your calculation quite a bit, and actually I would first use SingleZetaPolarized at least to get a roughly ok structure, then you can refine it using DoubleZetaPolarized.
You can first disable stress optimization and just get the forces down to small amounts, because of the Si-C bond lengths in particular, and then optimize forces + stress as a subsequent step. Or, if the suggestions above make it run faster, then keep it as it is, then you don't have to run twice... But keep in mind that 0.1 GPa is a quite low stress threshold, it may take quite some time to get there. So at least for the SingleZetaPolarized run you can use a bit relaxed accuracy, like 0.04 eV/Ang for forces and 1 GPa for stress.
Seems you changed "interaction_max_range"; don't do that, it's not a useful parameter, set it back to default.
Finally, I can't help noticing that you run the calculation in serial. Not knowing what license you have, that may be your only choice, but you could speed it up 2-3 times by running in parallel over 4 processes if you have a quadcore machine, even on your desktop/laptop.
Good luck!
