getting access to more UARTs on micro controllers
There are 7+ mainstream STM32F0 MCUs with 8 USART interfaces. There are several high performance STM32F4/F7 chips with 4 USART + 6 UART (for 10 total) interfaces, and three dozen chips with 4 USART + 4 UART (for 8 total).

I am pretty sure you can find similar options from other manufacturers if you stop looking for DIP package. The adapters from LQFP to DIP are freely available and cheap.