Configure the new one while the old one's still up, then swap the cables over.

Not entirely true. The big OEMs like HP and Dell often use non-standard motherboards and PSUs.

Everyone should at least give it a try, if only so your decision not to is well informed instead of following cargo cult advice.

Where's your bar for "not too noisy"? I have a rack case and the disks are the loudest things in it.

Depends entirely on what you want to do with it. Mostly storage with some containers and a little light virtualization? TrueNAS Scale. Mostly virtualization with less robust storage features? Proxmox.

Are you absolutely married to the idea of ESXi? Because it sounds like TrueNAS Scale might be a better fit for your use case. Most of your requirements seem to be centred around storage and it can handle backing up to a remote out of the box.

TrueNAS Scale is a good choice. Good storage features plus containers and some basic virtualisation.

In the wall I have a TP-Link Kasa KP115 so I can monitor the power draw of the rack. Then power goes into an APC SUA1500RMI2U which feeds a vertical PDU. My servers and router connect to the UPS using NUT so they can monitor the battery state and shut down when required.

Depends entirely on how you set it up. The 1gb/tb is a broad rule of thumb for running ZFS, if you're running ZFS you're probably running with no cache at all. No idea if your software exposes ARC hit stats, if not you can run arcstat.

On the other hand, running hardware raid is practically free because all the parity calculation happens on the RAID controller so you can spec out much lower end hardware (but won't be protected against bitrot).

Run TrueNAS Scale on bare metal and virtualise or containerise HA on it.

If you don't like paying for software you should probably avoid Unraid too.

Thanks. I vaguely remembered reading that post before but I couldn't find it last time I went looking.