This is cool!

I looked for this in the past. This is the main reason we bothered with mailchimp/hubspot -- simply the ability for nontechnical marketing people to put together nice emails, and the trust that we won't need an engineer to troubleshoot email formatting on their behalf. I remember trying some OSS tools at the time (8 years ago?) and there were some templates we used but then when we wanted to modify them, the broken-ness of email html/css standards made it really hard to test.

I know the standards and practice around this are a moving target, though, so I hope you can find a model to sustain and expand this, without charging for delivery/contact list numbers like MailChimp or other incumbents.