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.

Don’t use Outlook for non-Outlook email services. Problem solved. I am required to use Outlook for my work email, but I keep all of my personal email separate, accessed with FOSS email clients.

Care to elaborate on the significant downsides?

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.

No.

personal

Decent used laptops are quite affordable. I recently scored one on Ebay for under $100. It runs Linux and everything is snappy.