I recently started to write newsletters about Hamburg (https://alsterrunde.com). Before doing that I also considered Substack but seeing that I have to pay 50$ for just using my domain was a no-go. In the end, I ended up with my friend's hosted Ghost solution (https://www.synapsmedia.com).

If you already have a Substack, you can also easily migrate to Ghost. Since it is open source, you can also host it on Hetzner, if you want to manage it yourself.