So I commute year round in Calgary and that means cold and dark. I can't seem to find a tail light that isn't disposable garbage and will actually hold up to a few years of use.
Yes you can basically buy tail lights in bulk off Amazon - but I hate tossing dead junk in the trash. I would rather pay more for a light that lasts.
My perfect light would use replaceable 18650 (or whatever size) lithium ion cells, or at least have USB-C with pass-through charging (so that I can run it off an external battery in a pinch).
For front lights I use Outbound Lighting and they are perfect - long battery life, great performance, pass-through charging, works in the cold. Unfortunately Outbound doesn't make tail lights. I've begged.
I've been using a Garmin Varia for the past couple years while road cycling and love it. The built in radar might be a bit of overkill if you're not using a bike computer while commuting, but I've found it super useful.
I love the Garmin idea - especially because they are local, but I really don't need the radar functionality. I don't even own a computer, I just use my (Garmin) watch for tracking rides.
It's quite likely that you'd be able to connect a Varia to your Garmin watch if it's a reasonably recent model. Then you could get radar alerts as vibrations on the watch.