

I don’t have an answer for you, but a dev for that DigitalDiary app posted earlier today looking for feature input and saying they are going to open source it in the next 2 months or something like that. I don’t have a link to the post, sorry, but I just thought it might be helpful to know about
I think you’ve put more thought into how to get started than many others would! You have a pretty good plan from what it seems. My thoughts from each section below.
Hardware: I’m partial to Crucial and Kingston for storage that is affordable and dependable
OS: I’d probably spin up a Debian install if I were in your shoes and run my services using docker-compose files. It’s a quick and easy to get up and running, and despite the ease, there is still the option to do a lot of customization when you want to, and that will make it easy to learn more at your own pace and leisure.
Services: For the CalDav portion, I’m really liking Radicale.
Security: PiVPN is what I’m running on my actual RPi along with PiHole, and it was a super simple setup. I connect via Wireguard from any of my other devices.