Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

There's a documentary that's more than a decade old that now only exists on Amazon Prime. I have searched everywhere for this but the website (and thus DVDs being sold) doesn't exist any more, there are no torrents, no second hand DVDs for sale that I can find online..... I'm OK with buying the doco from Amazon (albeit not happy about giving Amazon any money) but once I've bought it I don't want to be locked into Amazon's infrastructure (I do not trust streaming services to keep such niche videos available indefinitely).

So how would I go about downloading and removing DRM from Amazon videos?

I read an old post about mp4decryptgui but that hasn't been updated in 3 years so I don't know if it works any more.

The only other methods I've found involve buying expensive Windows only software (I'm on Linux but can dual boot if absolutely needed).

Thanks!

(Also please excuse the freshness of my account - for some reason Kbin threw an error every time I tried to post using my usual account.)