I heard a justification for China's annexation of Tibet was that tibetans were slaves under the theocracy of the Lama. I naturally take it with a grain of salt. So I pose the question above. I'd prefer an answer relating over the whole history of the theocracy, but an answer relating to post-qing tibet would be great too!
I answered this before here, albeit with a rather long history of the Dalai Lamas and society on the Tibetan Plateau in general.
The long and the short of it is that there isn't a whole lot of obviously unbiased data about Tibet proper, so I use Bhutan as an example of what Tibetan society probably looked like. While I suggest taking a grain of salt with my response because Bhutan is not Tibet (which the Bhutanese are quick to point out), they draw most of their cultural, religious, social, and historical basis from their northern neighbor, so I think it's at least reasonable to assume there's some similarities with the societies.
The other point which is quite relevant is the problem of Orientalism when using rather common English terms like "slavery," "theocracy," and (other common ones you didn't use) "feudalism," or "caste." Those are traditionally broad terms which can only apply to non-European systems broadly. They usually come with a lot of historical baggage used by the speaker. So a Chinese source telling a Western person that Tibetans lived under a theocratic feudal society with a rampant caste/slave system, is only true in the broadest sense.
I encourage you to read my detailed response and look into the question further from published Tibetologists interested in history and not politics.
And if you have any other questions afterwards, feel free to ask here.