My drivers can do 1.2v before things get too hot with just one fan on them. If that doesn’t push your steppers you need need to find the issue. Your steppers should get hot with that much power, ours do. Even though your steppers are spec’ed as stronger, for some reason they use less power?
Did you loosen up all the bolts that control tension? I would find a way to put some belt on there hand hold it to verify it is actually going the right distance. These things would be so much easier if I could just swing by and have a look. Trouble shooting sight unseen is rough.
Is your z axis stronger, since it is only 1 stepper?