We recommend a simple USB Microphone or USB audio input interface. They have the necessary buffering and timing to correctly record audio with proper channels and bit depth.
Using SAR ADC would require precise timing of samples which is not possible. It is rather used for other analog measurements like resistor ladder keypads and temperature probes.
You can use I2S or PCM input with Le Potato. However you need to write a device tree overlay to enable the pinmux and enable it via the libretech-wiring-tool. This requires technical knowledge.