This might be controversial here. But if reliability is your biggest concern, you really can’t go wrong with:
A proper hardware RAID controller
You want something with patrol read, supercapacitor- or battery-backed cache/NVRAM, and a fast enough chipset/memory to keep up with the underlying drives.
LVM with snapshots
Ext4 or XFS
A basic UPS that you can monitor with NUT to safely shut down your system during an outage.
I would probably stick with ext4 for boot and XFS for data. They are both super reliable, and both are usually close to tied for general-purpose performance on modern kernels.
That’s what we do in enterprise land. Keep it simple. Use discrete hardware/software components that do one thing and do it well.
I had decade-old servers with similar setups that were installed with Ubuntu 8.04 and upgraded all the way through 18.04 with minimal issues (the GRUB2 migration being one of the bigger pains). Granted, they went through plenty of hard drives. But some even got increased capacity along the way (you just replace them one at a time and let the RAID resilver in-between).
Edit to add: The only gotcha you really have to worry about is properly aligning the filesystem to the underlying RAID geometry (if the RAID controller doesn’t expose it to the OS for you). But that’s more important with striping.