I'd love to know where to start (Discworld) as a old school hard scifi reader.

For the science fiction side of Discworld, check out the "History monks" path:

• Small Gods, Thief of Time, Night Watch

I personally include "Pyriamids" in that series, even though it technically doesn't have any history monks in it.

At that point, if you enjoyed the cop stories in Night Watch, you're in luck, beacuse:

As a science fiction fan, keep in mind that, while Discworld has consistent "science" rules, they're still intentionally silly. It is called "Discworld", after all.

But if you accept that Discworld is a shared delusion, with no explained origin, after that it's effectively hard science fiction. In my head canon, Discworld is "The Matrix" after a few centuries of missed maintenance patches.

Edit: After those, if the style appeals, the next to check out (for a science fiction fan) is probably the "Industrial Revolution" sequence.

I personally save "Moving Pictures" for last when I read theough it, because I'm impatient to get to the Moist Von Lipwig stories.

Also, the earlier "Death" stories have a sci-fi horror vibe.