The box in the garage is the NBN node. You will need to arrange for an internet connection with a provider if not yet done. It sounds like you then have Ethernet ports around the house so an Ethernet cable would come out of that node and into a port that should be near it

What burn rate did you create the DVD on? If you did it at x16 your reader won’t be able to read it

It could be possible whatever is on that side of the laptop may not be operating and so is not generating heat.

Have you tried taking out one set of the memory, turn off XMP and seeing if that solves the issues? Even though the brand, timing and model is the same, the fact that it has different manufacturing dates and die may cause some issues. Did you also check your motherboard’s QVL for memory? AMD are picky with memory and from memory has issues with Samsung die. Check your motherboard’s QVL and see if your memory model is listed

Have you checked that you have Secure Boot and TPM switch on in BIOS?

You mentioned you changed cables. Did you buy new ones or just used whatever was lying around? What are your monitor specs?

Just get a USB drive. Cheaper and does what is needed

What type of SSD do you have. If it’s NVMe, the issue with the old PC could possibly have been the motherboard PCIe lanes and not the SSD or GPU