The Adafruit HAT was out of stock earlier this year so I ended up with a USB-to-serial for everything but PPS (using GPIO) setup.

It's not the prettiest but works brilliantly.

Now just waiting for Raspbian to respin to be Bullseye based so that I can update to a version of chrony with NTS support.

You can just build chrony yourself.

(And I think you can upgrade RpiOS to bullseye right now. It's not "officially" out but it seems to work anyway.)