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Arduino boards typically don't have a DAC, but it's a common enough feature in microcontrollers that I figured I'd mention its absence.

Arduino Uno no, since that uses a chip that is over 8 years old. But a modern 8bit AVR which cost under 1USD in single units, like Attiny3214, has a DAC.

"like Attiny3214, has a DAC"

and is also not supported by any Arduino libraries to my knowledge, even with additional board support packages.

For DAC and good Arduino support use the Teensy 3.x

You can bit-bang I2C easily enough, as long as none of your slave devices have an SMBus-style bus timeout feature. Might be a bit hard for an AVR-class CPU to keep up with those without some attention paid to performance optimization.