For Thanos, you can (mostly) just copy the blocks from the existing s3 bucket into Mimir's bucket. There's some cleanup prep around external labels, but nothing too difficult, and the process is well-documented. I just finished migrating a year of Thanos metrics into Mimir without issue.
> I trust Google SRE to store my data long term far more than I do
I don't trust Google (that much) to keep my account active before my death, as their automated tools can shut down my account anytime and leave me with no access to my data.