Higurashi no Naku koro ni.
Not even going to say much about it, it hits on most of your specifications and it’s wonderfully horrible. Though closer to 90’s style animation than 80’s. If you wish to inspect it further then follow the link.
Mushishi
Not horror of any kind but it hits high on the scale for slow flow stories, scenery and landscapes. It’s rather new but you couldn’t tell by the animation style. Highly recommended.
Serial Experiments Lain
Slow, weird, oldish, much of the story is actually told through environmental clues and atmosphere. It’s not an “enjoyable” watch though but it is intriguing. The kind of series that you might get a lot more out of on the 2nd watch a few years later. : )
Psycho-pass
This one kind of falls outside everything you’ve specified but for some reason it was the first title I thought of when reading your post. Don’t even know why, but it’s a really good series and thought I’d mention it so you could at least look it up and figure out if you think it might suit you or not.
So far that’s mostly been anime from the noughties and later, I have not seen an awful lot of series from the 80s and 90s much less anything that would directly fit your criteria. I believe the oldest series I’ve seen is Bubblegum Crisis which is good but I’m not sure how it fits your preferences. I generally try to steer away from the more famous series and movies as most people have already seen them, anyone can recommend you a handfull of Gibli movies so that’s generally pointless but have you seen the old Berserk series?
Also I second the recommendations already made by @Satanwee and @Madway, especially Madoka Magica, if you’re looking for a great soundtrack.
Oh, oh!!
I just spotted another series in my list that I have to make an edit for.
Kemono no Souja Erin The art style and especially the intro to this series might confuse you for a moment. But this series is by no means for children. The story is slow paced and full of both heartwarming happiness and tremendous tragedy.
Which reminded me of From the New World also more of a tragedy series than horror. I don’t like hyping media up for people as that tends to detract from the experience and at this point I think I might have strayed entirely from the preferences you stated but there’s a lot of beautiful landscape shots in it. It’s good though, really good… you should watch it. : )
Just one more recommendation.
This isn’t an anime but one of my favourite movies ever After the rain a Kurosawa samurai movie. It’s a pleasantly slow and mostly peaceful story of a ronin and his wife stranded at a country in waiting for the rains to stop and attracting the attention of the local lord and his entourage.
