SOLVED
Start with the basics, right?
Problem was the starter. Thought I was jumping the starter solenoid right, I was not. Starter was original & just gave out.
New one installed, fired right up. Sounds much faster as well.
Install was a bit tricky, there is a bracket on the front of the starter ...

Picked up a beautiful 1993 960 wagon a few weeks back. Car runs & drives perfect. Well, did. I've put a few hundred miles on the car just fine.
Drove it to my shop yesterday just fine. Went to leave a few hours later for lunch & the car wouldn't start. Almost seemed like maybe battery but all ...