Is it not possible to use an I2C DAC?
Or switch to a SAM/SAMD based board since they have built-in DACs? I have not personally used this feature so take this with a grain of sodium-free salt substitute.
https://www.arduino.cc/en/Reference/AnalogWriteResolution
The Zero has the following hardware capabilities:
10 pins which default to 8-bit PWM, like the AVR-based boards. These can be changed to 12-bit resolution.
1 pin with 10-bit DAC (Digital-to-Analog Converter).By setting the write resolution to 10, you can use analogWrite() with values between 0 and 1023 to exploit the full DAC resolution