The SN differs by area. Different area has different ONT password. You can even backup your ONT config save it somewhere safe than upload back to your new ONT which now can easily be bought online.

They place the guide video now all over Youtube teaching users how to replace the ONT by yourself.

I usually disable TR-069 if I'm supplying my own equipment. It's a potential backdoor for others to gain access to your equipment.

MAC ID of ONT as well besides PLOAM password? Since they implementing universal ONU it can't be identifying Huawei ONT ID because need to allow other branded ONTs to coexist.

There must be a certificate or counter to uniquely identify the connected ONU/fibre modems out there to create a table.

If every time you switch ONU, it can't be allowing certificate/firmware updates everytime because the OLT won't be able to keep up with the list of connected ONUs connected to it unless it locks down a certain detail for each of them.

A OLT which accepts universal ONUs will need to read a certain international identifier from the respective ONU something like MAC ID for example or some sort of device ID before it starts pushing out the OTA update auth certificate to approve the ONU.

Unfortunately when you want to run a datacentre from home without proper lighting arrestors.
No pain no gain. At least you learned quite a lot about TM's GPON setup which the knowledge is worth so much more.