how can I convert it to I2C
There is no easy way to do this. The hard way is to have a micro controller doing the conversion but that defeats the point.
Or just keep it stupid and go with all digital and a cable for each one
Yes but that is not stupid, the sensor is made to communicate over a distance I2C is not. That is the sensible answer.
So I will need all the digital pins that I can get
There are lots of ways to get more digital pins. Shift registers or port expanders are easy to use.
Look at the MCP23S17 chip, that gives you 16 I/O lines.