I think T3 and Salvation were just badly written, badly cast and generally badly made.
I don't know what was or wasn't good or bad about Genisys because it was so unpopular that it's still never shown up at my library or on any streaming service I've been using, so I still haven't seen it. Sounded mildly interesting to me, but apparently the world disagreed...
Dark Fate, on the other hand, popped up on amazon prime last week. I quite enjoyed it. As much as some people apparently want to rage about it 'just rehashing the first movie', I thought it was the first time the franchise actually managed to come up with an interesting new idea in a long time. I like the implication that the war has changed and the enemy has changed, but not by that much, and that there is always a new leader to stand up and pull humanity together, and I love the idea of adding bio-enhanced humans to the franchise and of exploring a Terminator's free will. And Linda Hamilton was absent from the films for far too long - she's as much or arguably more the real star of the franchise as Schwarzenegger. The only thing I didn't like as much was the two-for-one terminator model, which just screamed 'trying too hard'. But I'd watch a Dark Fate sequel happily if one happens.
As for the rest of the ideas, the only thing I've seen mentioned that sounds even remotely interesting is the idea of Skynet just moving up the war altogether. If I can't win in the future or eliminate my enemy in the past, then I'll just eliminate human civilization in the past instead. That's an intriguing premise (which the future war just isn't) and a logical escalation from the basic story parameters. But it does carry the problem of having to figure out how the heroes could possibly beat a super-advanced robot uprising without their leader (who's still a kid), and with woefully inferior technology.
Everything else just sounds like more of the same, which is exactly what we've already had more than enough of for this franchise.