Solved New build - old licenses step by step

Just to complete the thread, I ended up buying a key from Digital License, they claim the key is transferable to a new system, which is a plus if I need to reinstall or change parts. The license is digital, I did use a restore point made before I activated Windows and it activated itself with no problem. I only have a local user on the system, no Microsoft account at all (installed Windows prior to connecting the Ethernet cable).

Same with Office - bought a key from the same place. Now, the key being cheap, it's probably a volume licensing key. So my Office install is Pro 2024 LTSC. It's activated and working. The seller claims it to be transferable, but it is not clear whether this will actually work. However, I do upgrade my PC once in several years, so the minimal price of the key seems to offset that risk.