Connecting a 2012 27" iMac to a brand new 27" iMac

You should be able to use the 2012 27" as a display for the new iMac, Apple explains this https://support.apple.com/en-us/HT204592 and advises the connections to use. You may need a Thunderbolt to USB C adapter such as https://www.apple.com/shop/product/MMEL2AM/A/thunderbolt-3-usb-c-to-thunderbolt-2-adapter

As far as accessing the old computer's HD while in TDM, that I don't know about, perhaps someone else can weigh in.

To connect your Firewire 800 device you will need a Firewire 800 to Firewire adapter and also a Firewire to Thunderbolt adapter such as https://www.apple.com/shop/product/MD464LL/A/apple-thunderbolt-to-firewire-adapter

Good luck.

You could set up file sharing over your network from one Mac to the other. This would be only as fast as your WiFi or Ethernet wired connection, but should remain active while one of the Macs is in Target Display Mode.

I do not know if you could operate the Mac in FW Target Disk Mode, which might not be that much faster anyway, at the same time the Mac is in Target Display Mode. Since the Mac must first be booted up into Target Display Mode, I don't think that it would work if you have already booted into FW Target Disk Mode.

The better performance option would be to get the appropriate adapters for the Drobos to connect directly to the new machine, but that might require daisy-chaining two adapters to get you there...

https://www.lifewire.com/mac-usb-c-to-old-peripherals-4122273

"At the moment, there's no direct Thunderbolt 3 to FireWire adapter

available, and we doubt one will ever be made. However, Apple makes a

Thunderbolt 2 to FireWire 800 adapter, which you could combine with the

Thunderbolt 3 to Thunderbolt 2 bidirectional adapter mentioned above."