Microcontroller and design for multiple displays
I do not see a problem, you just have not found a suitable MCU yet. Even if a MCU has a single SPI interface, you can use all the other GPIO pins for chip selects, connecting dozens of SPI chips. Many MCUs exist that have 4 separate SPI peripherals. It seems the one I am now working with has actually up to 6, but most likely these smaller packages I am using do not have access to all of them, so larger pin package is required.