A suggestion for the Zulip team: email is the hardest part of setting up any self-hosted solution, and Zulip is no different. You covered a lot of bases in your email section: https://zulip.readthedocs.io/en/stable/production/email.html
However it's still too difficult for many people to setup an account with an ESP, get the SMTP creds, and configure Zulip to use it correctly. It's a big step for people newer to self-hosting. I would suggest meeting somewhere in the middle for those people, by offering a paid self-hosted tier that handles some of the "harder" stuff for them. You can see how the popular 3CX phone system does this for inspiration: https://www.3cx.com/docs/fqdn-management-allocation/
All 3CX accounts receive a FQDN and confirmation emails are routed through this managed service, no configs needed. You could charge perhaps $80/yr for this tier, so customers wanting to host their own data can do so, but have SMTP and DNS managed for them by providing them with a Zulip subdomain, like Slack does.