Sounds like a fascinating project.
It sounds like the majority of signals are driven by one ATTiny85 and received by a neighboring ATTiny85 about 12 inches away, with a good ground connection between them.
Those signals can be directly connected.
If I were you, I would go ahead and get differential transceivers for the 10 meter distance.
If you use daisy-chain SPI, then you need 3 differential line drivers at one end of the cable and 1 differential line driver at the other (and the corresponding line receivers to match); plus GND and +12VDC.
If you use a SN75176BP as a line driver and another one as a line receiver for each SPI signal, that's a total of less than $8 for those 8 interface chips.
(You need fewer line driver chips if you somehow daisy-chain TTL UART serial, since it uses fewer pins, but requires each chip to have more precise baud clock).
The software should not even be able to detect the difference between a simple wire from the output pin of one chip to the input pin of the next chip, vs. a line driver, some twisted pair cable, and a line receiver between those same two chips running the same software.
Many people use the rule of thumb that if a bit time is less than 10 times the time it takes for something traveling at the speed of light to make a round trip from the transmitter to the receiver and back, then you must use differential signaling with line termination.
At 1 MHz, that gives a (1-way) cable length of 15 meters before you must use differential signaling with line termination.
So using the differential line drivers is probably overkill, but I'd rather have the first prototype work rather than cut corners on 100 things that I'm 99% sure probably won't matter, and then spend a lot of time debugging trying to figure out which one or two of those 100 things turns out that it does matter.
In addition to distance, loading might also be an issue.
In particular, the "CLK" and "/CS" pins from the SPI master apparently need to drive every SPI slave.
You can't really expect a single MCU output pin or even a single RS485 line receiver to be able to drive 200+ other chips.
A line driver or other buffer every 10 or so chips is more than adequate.
"What if the SPI signal lines are very long?"
(this experiment completely neglects the effects of interference from other noisy devices).
AN10364 "Transmitting the I2C signals as differential signals using RS-485 hardware"
(Like SPI, I2C signals were never intended to be transmitted long distances over wire cables.)
"Maximum Unterminated Network Speed Calculator"