I am enquiring to which down tube protection systems people prefer on their carbon frames.

kydex

For me its part of a defective nobby nic.
It offers the best protection also doubles as a clamping point if you have a frame holding type bike rack.
If you have worn out tire, it is free as well.
You can also cut it to a specific shape and with a good pattern tire it is not even that bad looking.

clever

I've been using some generic AliExpress chainstay & downtube protectors. Decently thick, but not heavy, and good adhesive backing. Best of all, ridiculously inexspensive!
https://www.aliexpress.com/item/3256802362432309.html?spm=a2g0s.8937460.0.0.35f42e0e1GGAtG