Any other time I could name ten different bands for you to try, but lately I've had nothing but doom and gothic tinged metal on my brain. I don't think that would scratch this particular itch.
Maybe look into some modern Enslaved? I know they've been around for a billion years so they're as big as niche metal bands ever get, but they fit most of your other criteria.
Try something like this first.
- Two singers: their original black metal shrieker and a guy that sounds a lot like David Gilmour. The contrast is like night and day.
- Fast as fuck overall, but...
- Their songwriting is complex, with tempo changes all over the places.
- They're technically impeccable; they manage to push the envelope without devolving into prog wankery.
If nothing else, it'll narrow the field a bit. :P