To make it seamless so you can still Thunderbird, someone made a Docker image of it here: https://hub.docker.com/r/kebles/wanderbird But, you can probably find a newer release somewhere newer than 4 yrs old like this one. :) The point is, if you are wanting to keep it in the Thunderbird umbrella, then it’s most likely been Dockerized.
I’ve Tried Cypht recently, but if you are using Gmail, it has a conflict there so it won’t work out of the box without some extra work I think.
You could always use ntfy.sh if you are wanting to keep it light weight, I know there seems to be a heavy following and happy community with it. I personally use Gotify which has been nice and easy to use and just works for my needs. :) I looked at the shoutrrr repo and it seems to be either abandoned or just no longer updating maybe because their is no need to in the Dev’s eyes. They also develop Watchtower which hasn’t been touched in about 2 years ago. I have never had any issues with Watchtower so I think it may not need much maintenance. I do see though that they are working on a new project: https://github.com/containrrr/shepherd but it’s also a bit stale.