Yoda is every bit as strict about the Dark Side in the OT as the Jedi were in the PT.
Not true at all.
“If once you start down the dark path, forever will it dominate your destiny.” It doesn’t get more absolutist than that. He also believes that Luke should kill those who use the Dark Side. Luke proves him wrong on both fronts. The Jedi in the PT have more explicit rules, but the underlying absolutism is the same.
It’s vaguer than that, there’s still room for interpretations of the dark side to grow beyond what we see in those films. He’s basically just saying once you go dark, you don’t come back, which of course Luke proves is wrong by turning Vader. When he talks about fear and anger there, I take it to mean he’s saying be careful of those emotions, he’s not necessarily saying to repress them.
In TPM the Jedi council doesn’t want to train Anakin because he’s afraid. Which is dumb. Not to mention their views on love and attachments, of course. Problem being that there’s no one in the movie to prove them wrong, but there is someone who proves them right.