yeah...I have found
YouTube to be food for short documentaries. but the get exhausted easily.
A lot (most, maybe all) the Time Team episodes are up there; Prime only carries something like four or five seasons. Michael Wood docs are all over YT, most re not available on Prime (or on the BBC website, apparently); loads on Roman Empire (including a bunch on Pompeii et l), Bettany Hughes on ancient Greece, and the more you watch, the more previously unseen ones pop up on recommended vids on Roku. :^)