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.

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?