STM32 SPI DMA timings between data frames and multiple DMA profiles
The issue is that the stock SPI DMA stuff ST provides doesn't actually fit your use case here. When you use their SPI driver, it assumes you want to send the data as fast as the bus will support. This means back-to-back transfers, which is what you are seeing. 

See if you can look through the reference manual, find how the SPI Tx signal is triggering the DMA, and see if that trigger can be moved to a timer-counter. This is usually possible on full-featured microcontrollers. Then set up a timer-counter to trigger on the period you are interested in.

If you can use the ST HAL driver at all, it'll have to be rewritten to do what you want.