Nextcloud is feature-rich, but a little slower and vastly more complicated than a CalDAV server like Baikal.

Use a hardened-by-default OS like OpenBSD, HardenedBSD, or something similar in Linux.

OpenSMTPD. I run a private mail server for family, and it works flawlessly. No maintenance aside from backups and occasional OS upgrades. https://poolp.org/posts/2019-09-14/setting-up-a-mail-server-with-opensmtpd-dovecot-and-rspamd/