Yeah, there's not really a great way to do this. I have a similar problem since I have books from a variety of services/countries.
On Android devices like the ones from Boox it is possible to have multiple copies of the Amazon Kindle app (or Kobo) with different accounts, but it is still kind of a pain and is not straight forward. You may occasionally run into issues with that as I have. Going DRM free is the best path if possible. Personally now I use a Kobo, keep all my books on a server with Calibre-Web and then sync the Kobo to that.
Kindle is not a great choice here IMO.
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