WD Blue 6 TB (WD60EZRZ), slowly failing. Read errors are getting worse with each read, likely a weak head. Partial clone done, but critical files are missing. Considering Blizzard Data Recovery or Rossmann Group. Any experience with either for head swaps?

Both will be fine - If it's still reading it may not even need replacing. Rossmann has a longer turn around time if I recall , especially at the moment. Blizzard is also likely to be cheaper if that's a concern.

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Data Recovery Services in the UK.
https://www.usbrecovery.co.uk/