You have a certain number of usecs between interrupts. Do you need to send 8 bytes between every interrupt?
because I thought there is more than 1 interrupt just to generate those 8 bytes.
Otherwise 333 us per message, 3 messages per ms, 24000 bytes per second makes 240000 baud which does not fit well with a 115200 baud limit. I think there must be at least one wrong assumption there or somewhere.
I'm not asking if you need 8 byte data packets. I'm asking how often you need them. There might be a data management solution, when I calculated before there were gaps big enough try to fit one in.