I finally set up a newsletter I am relatively happy with. I used EmailOctopus (an Indiehackers project) for list management and sending actual campaigns. It is not the best and ideal, but it is not overloaded with unnecessary features.
One of the issues I've had is the ugly sign up forms. To be clear, EmailOctopus does not solve that, but! This is the first service that provided an easy enough API for me to implement on my website. It was very easy!
I am in the process of writing a post about it. As of now, I am happy with EmailOctopus. I am excited to have dealt with all the setup and getting actual newsletter writing.