Trying to recover a serial number or other unique identifier...

Hello Johannes,my name is Oskayra, I will be happy to help you in any way possible.

To connect a self-made PC to your Microsoft account, you need to obtain the PC's serial number. The serial number is a unique identifier used to identify the PC.

The serial number of a self-made PC is usually printed on the motherboard or case. You can find it by looking for a label with the text "serial number" or "SN."

If you cannot find the serial number on the motherboard or case, you can try to obtain it using the Windows Diagnostic Tool. To do this, follow these steps:

Start the Windows Diagnostic Tool.
Select "System Information."
Look for the "Hardware Identifier" section.
The serial number will be displayed in the "Computer Serial Number" field.

This kind of problem is very annoying to continue using the devices but with these kinds of solutions I hope you find the problem, I wish you luck and have a happy day.

Sincerely,
Oskayra.

Is it illegible or the sticker fell off? Where did you get the controller? The elite controllers register separately for device support, but regular controllers do not, so I'm not familiar if there is a way to recover the serial number of that device
if not identified on the device itself.

There really is no way to track it unless you have a serial number and its unique to the item.

Taken directly from Wikipedia

A stock-keeping unit or SKU is a number or code used to identify each unique product or item for sale in a store or other business.

It is a unique identifier for each distinct product and service that can be purchased. The usage of SKU is rooted in data management, enabling the company to systematically track its inventory or product availability, such as in warehouses and retail outlets.
They are often assigned and serialized at the merchant level. Each SKU is attached to an item, variant, product line, bundle, service, fee, or attachment. SKUs are often used to refer to different versions of the same product. For example, a retail store carrying
the game Guitar Hero 3 might have six SKUs, two for each of the three platforms—one with and one without a guitar controller.

I understand. The account's primary email address cannot be linked to multiple accounts or Xbox profiles because it's unique in every accounts. Your original gamertag with no suffix must be linked to a different email address. It is because, if you linked your Minecraft account to your Microsoft account (with gamertag that has no numbers) you won't be able to set or claim the same gamertag with suffix as they're technically the same gamertag and the numbers or suffix is just an identifier for each unique account.

I suggest to try to enter your phone number or emails or skype username if you had it linked to your original account here https://account.live.com/username/recover to find your correct account.