Another "future me" reply:

I found the SAML21, which I really want to test out.

We have done a first project using the SAML21 at work. We choose it instead of the SAMD21 mainly because of the second DAC channel, that significantly reduced the number of components required for a very basic port-expander like project. That board will very likely be used as a component in several following projects. We don't use many of the new features (compared to the SAMD21), but our hardware design does not prevent us from doing so. We still use the LDO for supply, it is good enough. Our experiences:

• The DAC is not just doubled, it is a completely different peripheral, closer to the SAMD5x/E5x DAC peripheral. So we reworked the SAMD5x/E5x driver.
• The ADC was also replaced, and again it looks more like the SAMD5x/E5x peripheral than the SAMD21 ADC. Again, reworked the ADC driver from the SAMD5x/E5x.
• The clock system is similar to that of the SAMD21, but sufficiently different to require some drivers changes.
• The SERCOM5 does not support DMA, so we had to rewrite our SERCOM USART driver to support interrupt-driven transfers in addition to DMA, and to use IRQ-driven transfers for SERCOM5. You should not think of SERCOM5 as a usual SERCOM, as it has too many restrictions. It is probably most useful as a debug console output, because it is available even in low-power modes. If you don't need all six SERCOMs, and power saving is not a big issue, avoid SERCOM5 and use its pins for other peripherals or for GPIO.
• No experience with CCL, Op-Amps, RNG, AES
• No experience with power saving

To explain the last point: Most of our hardware either runs from mains power or from high-capacity batteries. When running from batteries, the power required by the microcontroller is typically in the noise floor compared to the remaining hardware (touch screen backlight, solenoid valves, pumps, sensors with heating elements, radios, ...), so its power consumption simply does not matter.

Power saving becomes interesting if your hardware has to run from low-capacity cells (something like a CR2032), and is always powered. Maybe waking up for a few hundred clock cycles every minute or hour, then going back to sleep as quick as possible. In such scenarios, every µA counts.

But, as explained, our peripherals easily need three-digit mA at 12V or 24V, i.e. several Watts, compared to one-digit mA at 3.3V (Milliwatts) for the microcontroller. Switching off a single power-hungry peripheral easily saves two or three orders of magnitude more power than fiddling with the microcontroller's power management.

Alexander