If you’re looking for a self-hosted replacement for OneDrive, you’ve come to the right place.
This should be more than sufficient if it’s just for your personal use.
Ubuntu Server will be perfectly fine for Nextcloud.
I don’t know that there’s a major difference from a security standpoint, but I run it in Docker personally, and I wrote a guide for it if you’re interested.
I can’t speak to ZFS, but RAID-5 doesn’t offer more reliability than RAID-1. Both tolerate only one concurrent failure. RAID-5 will give you more usable storage at a sharp cost of write performance.
I personally use Apache, and that’s what the Nextcloud docs also recommend. Many people use Nginx.
It’s really not too bad, but understand that you will need a working knowledge of DNS and firewalls and a local DNS server if you want this (or any other self-hosting) to be a smooth process.
If you need a better firewall, have a look at pfSense.