See RP2040 manual section Section 4.2.6.4. 'UARTRTINT', which describes the receive timeout interrupt. I believe the timeout is fixed at 32 bit times.
As @sawdust's comment indicates, the ISR for this would abort the current DMA transfer (or perhaps not, depending on your design).
Regarding "UART interrupts don't fire if a DMA channel is active with the UART": the timeout interrupt should be unaffected by DMA operation.
Of course you need to disable the specific rx and tx interrupts within the UART mask register, but enable the UARTRTINT interrupt, and enable the UART interrupt in the NVIC.
