Uart rx: whats the best solution Receive unkown size of message

Each byte reception generates an interrupt, you process byte that into a buffer in a stateful fashion, when you have a complete packet, or line, or however you delineate content, you pass that off to a worker task to process/digest. If you don't like interrupts, have a circular DMA buffer, with adequate size for your sweep frequency, and have a worker task pull the data for processing.

https://stm32f4-discovery.net/2017/07/stm32-tutorial-efficiently-receive-uart-data-using-dma/