Is it more or less the same problem as running a GNU Mailman listserv in 2022? That is to say, a fairly hard problem of staying on top of security patches, avoiding getting marked as spam, and avoiding getting put on the big email provider's badlists. In theory, the problem seems simple: replace a manually-managed BCC list with an alias to said list, so it's less hassle for the user/sender. Maybe the answer is an email client plug-in that just autofills BCC to-addresses? Assumption would be the user's sending this email from a known provider like GMail or Fastmail.
I imagine it would be easier for the completely self-hosted email system if signup gave receivers a from-address they could whitelist, if their email provider allows it. Maybe you already do this.
Edit: I'm also assuming email is sent one-way to whoever signs up, maybe with a link to the dynamic chat server/app/platform of the moment. Slightly different from a GNU Mailman list, which enables back-and-forth conversation.