Nice project, I'd totally set this up for myself as social media replacement. Please let me know once your MVP is ready. My main concern would be to overwhelm recipients if they'd ever get included in multiple people's newsletters.

I've been building on the opposite end of this. It's an open-source form submissions forwarder. I built it mostly because I wanted a service thats transparent, with usage based pricing that I could use economically for my low traffick sites.

Sometime this year I want to add docker-compose to make it easier to self host. Currently it's all manual. https://simplecontactform.org