The centre being all black would have added a nice touch

LGR has a nice review of this and a similar machine: https://piped.video/watch?v=6bODiZ5bP84

I’ve recently done this on one of my machines, these were the steps I as far as i remember:

• backup!
• add the new disks to the machine
• partition them like the smaller disks
• add the new disks to the array(s) (as members, not just spares)
• take care of bootloader things while the raid is syncing
• your raid should have 4 devices in sync now
• fail and remove the smaller disks from the raid one by one
• resize the partitions on the bigger disks to maximum
• grow the MD raid to the new size

If you use luks and/or lvm some additional steps will be needed to grow them as well.

If your system can hotplug disks there’s a chance to pull this off with little to no downtime. :)

I can’t really tell you what to use, but from my personal experience - stay away from glusterfs and drbd. both have caused me serious trouble when trying to run them in a production setup. ceph seems to be pretty solid, though.