Cool you experiment with the difference between flour!

Idk if your starter is super strong.

If not. You could try to not only use 100% unbl flour but: give 50% rye flour and 50% unbl flour a try.

Maybe the t65 can develop more gluten if there is a mix in the starter?

The more water you use the faster your dough is raising. (You said you did 70% before with an other flour mix)

So maybe it can be your dough isn’t at its peak within 2 hours of bulking with only using t65?

If you do stretch and folds, is the dough strong like a ball after a few times? Because 60% hydration is not super wet I assume.

Do you see bubbles while your dough is bulking?

I’m always open to talk about bread, just so you know!