So, on PC, I use the same account for the different regional sites.
I’m from Germany, so german Prime account - when I use a VPN with an english location and go to the UK amazon page, and the log in there with my german account, I can watch all the free movies/shows that engish Prime users get. Same with the US version of Prime.
If I understand you correctly, the region of the Fire TV stick itself prevents this from working?
You could try sideloading the Android version of Prime Video, I’m not sure if regional content is still tied to the account region there, or if a VPN might work.
Sideloaded Android Prime Video will be limited to 1080P streams if that matters to you.
The Android TV app still recognizes the region - I get the stuff from the chosen country in the VPN, but when I try to watch it, there’s a region error.