Maybe consider mailbox.org. It is cheap, supports custom domains and is hosted in Germany.

I guess having a backup and error correcting file systems like ZFS or BTRFS will help you more long term. Sure, watch fir Smart values, but imho don’t go over board with tests. I do a extended smart test, rebuild/extend my RAID, check a quick smart test again and that’s it. Drives can die at any time, even if they were fine after a long test cycle. The 3-2-1 rule should save you from data loss.

If you are willing to consider commercial products, I can recommend Synology DiskStations (at least the plus series). Samba shares are quite easy to setup, you can use Synology Drive to sync a folder between workstations and Android phones which I use for Obsidian for note taking. They also have calendar options, but I use a hosted account at posteo for that.

If you want to stick to nextcloud but don’t want to host it, you could consider Hetzner Storage Share. It’s fully managed and worked great for me so far. But I only use it to share photos with others, so not all features.

Why you chose to open a port, if you use cloudflare? Couldn’t you use cloudflare tunnel in that case?