Hello Matticus,
Welcome to Microsoft Q&A!
I'm sorry to hear that you're unable to log in to PS5 with your Microsoft account, as it was linked to PS4. Unfortunately, Microsoft does not provide a self-service unlink option for PSN accounts. You need to do that from the PS4.
You can try to contact PlayStation Support and request an unlink. Explain that you no longer have access to your PS4 and need the Microsoft account unlinked so you can use it on PS5.
If regaining access to the original PSN account is impossible, your only option is to create a new Microsoft account to use with your new PSN account on the PS5. Your existing game progress and purchases tied to the old Microsoft account will not transfer.
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