Oddly enough Season 8 is available through Prime all by itself .....Seasons 1-4 are offered through Amazon's IMDB streaming service viewable through Amazon. Apparently Seasons 6 and 7 are pay extra to view on Amazon (with the Shout! Factory logo) in the same playlist as the Season 8 I mentioned previously that's included with Prime.
Seasons 1-4 are also on YT in a playlist on FilmRise's channel there, on Tubi TV and on Pluto in their On Demand section. I know FilmRise and Shout TV have different distribution rights but it's weird that Season 5 isn't included with 1-4 which are free to view on multiple sites while also not being included in what Shout! Factory is offering through Amazon to view.
P.S. I just found seasons 5-8 all viewable for free on Shout! Factory's website. The picture quality isn't that great compared to the seasons FilmRise has made available. They also used the edited version of the opening which I think is the international one that omits the lady vocalizing. https://www.shoutfactorytv.com/videos?utf8=✓&commit=submit&q=silk+stalkings