UART Data Transfer Across Modular Boards
Could one microcontroller output handle the inputs of up to 10-15 microcontrollers?


The GPIO pins are CMOS, so they can handle many more than that, in terms of fan out. In fact, it is more than you can get from standard 1 unit load RS485 transceiver.

At 1Mbps however, the capacitance of the line comes into play, so a lot depends on the connectors you use.


  RS485 uses twisted pair which would be difficult to achieve on a PCB from what I can tell


Running traces on PCB parallel to each other would pretty much have the same effect as twisting wires.


  This would allow instantaneous transmission to all of the slave microcontrollers as needed.


Here comes the $100 question: what is "instantaneous" to you?

The simplest solution in your case would be to add a tiny schmitt buffer like SN74LVC1G17 at the output connector of each module. Then you can daisy chain your modules practically indefinitely.

The catch, however, in propagation delay, which for the chip above is about 3 ns. So, for the chain of 10 modules you are looking at 30 ns delay between first and last module. If this is instantaneous enough then just go ahead with UART and don't bother with transceivers or multiple wires.

Another catch is that your master module must be located at the beginning of the chain. If this is not convenient, you can put the buffer between Tx and pass-through line. It has 24 mA driving capability and only 4 ns propagation delay into 50 pF load. This should be sufficient for 6 ft line with 10 connectors.

BTW, differential buses have similar issue - you have to add termination resistors to each module but only activate them at the ends. Which means you have to keep an eye at each module placement anyway. Might as well keep an eye on one module instead of two.