Well, turns out my tensioner rod was really messed up. It was actually bent/bowed, presumably due to a previous owner trying to apply tension without loosening the tensioner itself. When I tried to re-apply tension after changing out the belt the rod just snapped.

Surprisingly my FLAPS had tensioner rods in stock from meyle. I took everything apart, put the new rod on the existing tensioner and re-installed everything. When I tried to re-apply tension the new meyle rod broke off at the bottom of the tensioner. The tensioner was already almost to the bottom of the threads and the belt still wasn't getting tight, which makes me think the tensioner and/or shock is worn out.

geez, all I wanted to do was change a belt on a car with 250k on it

I installed a new tensioner, shock and adjusting rod. Everything was easy to adjust and is in spec now. The belt tightened up as expected with plenty of room left on the tension adjusting rod.

Seems pretty obvious now that if your tension adjusting rod is at the bottom of the threads and the belt still isn't tight, your tensioner is probably finished and needs to be replaced.