Do you know how to use USART on STM32

You can configure DMA to work in cyclic mode, and with reasonable big buffer you can pull characters as often as you want with simple poll function. Just store index of last character retrieved and use DMA register to check how many characters it has to receive until roll over (AFAIR register has NDTR in its name), and process characters received since last call, then update index of last character retrieved.

Described DMA usage makes your poll independent from context you call it from as long as you prevent preemption. Then you can use RX interrupt to trigger poll call(s), or you can do it in other context (for example with some periodic events).

This is generally efficient if frames are long enough, baudrate is big and interrupt latency is bigger than reception time of single char.

But if data is received slow enough you can get away with processing it char by char current way, and using DMA may be overkill.

As @Chris Stratton pointed you can also just set DMA for single transmission, and wait long enough, change protocol or use other signal as "end of transmission" - and then process frame in DMA buffer.

This line looks suspicious:

UCSR0C = (1<<UCSZ01)|(1<<UCSZ00)|(USBS0<<1);

You probably want that to be:

UCSR0C = (1<<UCSZ01)|(1<<UCSZ00)|(1<<USBS0);