Obviously with a 320M motherboard you can not OC the 1600. So your 1600 is performing exactly as spec-ed.
Nonetheless, TWO things jump out when looking at this. The biggest being the 1x8GB memory. This is cutting your memory bandwidth by half. You are getting only 13.7 GB/s vs. like my 2x4GB doing 32.8 GB/s.
That is a massive hit. You are already using the slower cheaper 2133 speed memory and then to deprive yourself the dual channel option with a single stick, this is going to really hurt. On top of it, Ryzen does like slow memory to start with. Whether the cost benefit can be justified for 3600 or higher speed super high-end memory where the pricing premium get ridiculous is whole other debate, you want to be in dual channel memory operation, be it 2x4B or 2x8GB, even with cheaper slower memory this alone will make a big difference.
The second thing is that Sandisk SSD. It is performing as expected, but the that is some slow SSD. 219MB/s vs your HDD of 133MB/s is not really that great. Typically people buy SATA SSDs with expectations of ball park 400MB/s. Normally you might be able to get away with it, but you are compounding that with half the memory bandwidth. That is really going to make things hiccup.