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/

Grep. If you have the sources digitally and local, you can just run a search for a string.

If Hugo stops working down the line, you can compile your markdown with something else. Simple is king!

Not my experience at all. Once my OpenSMTPD mail server was set up 3 years ago, the only time I spend on it is updates (like 20 minutes per year). Setup did take some time, but it was totally worth it to me.