Hope this is the right board to ask this... Other than displaying gauges and warning lights, immo junk, what (if anything) does the cluster actually do for the car?

Can I just delete the cluster entirely and run without one?

In the long run I'd like to like to make my own custom dashboard by reading CANbus data and just get rid of the crap stock cluster, but I don't understand what else I might be losing/breaking by doing so.

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BEA 1.8T, 2004 Audi TT, IMMO3 if it matters. My cluster is shot (loses power constantly while driving, random/incorrect errors, needles only move when they want to, etc). I've immo-off'd the ECU, so it doesn't care what the cluster is doing and the engine runs like normal regardless.

I tested today driving around with the cluster physically unplugged. Nothing seemed out of the ordinary, car still started and ran fine, turn signals worked, AC good, alternator charging, etc. Also tried reading data from OBD while testing, only thing I couldn't get was vehicle speed (remained at 0). I could still get rpm, coolant temp, vacuum/boost, afr, etc.

The FR has CAN data for the cluster (pages 1662-1666) which seems to suggest the cluster should be publishing vehicle speed onto CAN, but also that it's responsible for coolant temp, so something doesn't add up, or I'm missing it in my machine-translations. In reading other threads people say that the cluster is the CAN Gateway, which if I understand correctly would mean a lot more problems for other systems in the car.  It's hard to test everything though, plus this era of car the CAN doesn't seem like it is used for everything. Haven't seen the ECU throw any errors yet but I'm worried that it's expecting data from the cluster which obviously isn't there.

Hoping someone on here has more experience with this?

Nameless is right. Set GGVFZG to 1 or 2 so ECU sees vehicle speed from can and you're good to go. Also, OE cluster acts as a relay for analog vehicle speed signal to the radio, AC, comfort module etc., so you should bridge "speedometer input" (blue connector pin 28) with "speedometer output 1" (blue pin 3) and "speedometer output 2" (blue pin 31) at the cluster connectors for everything else to work correctly. If you bridge these wires, ECU will see vehicle speed as standart, without setting GGVFZG. It up to you how you want to go about this.