Large transfers by UART

[ThinkTransit]

I have a modem connected to an ESP32S3 - 8MB PSRAM 32MB flash.

I need to download/upload largish files (~500KB) which involves pushing the data through the UART to the modem. @115200 I think this will take around 45 seconds.

Just trying to figure out what the best strategy to handle this is, to avoid 45 seconds of blocking code. Any advice appreciated.

Are there any practical limitations to the size of the UART buffers, can I make them 1MB each direction?
Does the UART code run in a different core, would transferring data from UART to PSRAM block main MP code?

I'm thinking that the best option would be to use large buffers and read the data from the buffer to flash in chunks, allowing the code to yield in between chunks.

Maybe using async streamreader/writer could help or maybe not because writing to flash is not async?

Thanks