But I don't have a Microsoft account (only ever used local accounts in Windows 10). How can I ask for all my data which is associated with my installation/license?

This is a very good question. I suspect that at some point someone will fire up their lawyers and find an answer.

Not only that but you can’t opt out from data collection at the moment.

Only the basic telemetry (which I assume doesn't include any personal data) can't be opted out of in Windows 10 home. Other privacy settings have to be opted in now. The recent "April update" forces you to say yes/no to all data collection settings.

Basic telemetry is HUGE. You can't opt out of basic on Pro either.

https://docs.microsoft.com/en-us/windows/privacy/basic-level...

There's enough information in there to identify a machine and user via correlation.

Here's a full analytical report: https://www.lda.bayern.de/media/windows_10_report.pdf

This! I don't use the MS account for my installation. Nevertheless, a lot of stuff must be sent to them about me.

My best guess would be that your data is going through pseudonymization and so you are no longer uniquely identifiable.

[1] By contrast to anonymization, Article 4(5) of the GDPR defines pseudonymization as “the processing of personal data in such a way that the data can no longer be attributed to a specific data subject without the use of additional information.” By holding the de-identified data separately from the “additional information,” the GDPR permits data handlers to use personal data more liberally without fear of infringing the rights of data subjects. This is because the data only becomes identifiable when both elements are held together.

[1] https://iapp.org/news/a/looking-to-comply-with-gdpr-heres-a-...

No, look at the actual law and you see that is plain wrong:

(26)

The principles of data protection should apply to any information concerning an identified or identifiable natural person. Personal data which have undergone pseudonymisation, which could be attributed to a natural person by the use of additional information should be considered to be information on an identifiable natural person.

I think the actual answer is that you send a snail-mail letter to MS, supplying proof of identity, details of your license and ask for all the PII that they have on you.

Go here: https://aka.ms/privacyresponse

I have no clue what information they'd need from you to answer that question, but at least you can get a response without having to send a physical letter.