Last time I checked, commandos could use an ability that allows a casted spell to be instant. Use it in conjunction with your big heal. Less squishy. And if you use a wz med pack on top of that .
What you describe as tankyness is not that at all. A tank stands there and soaks up damage. If a sage just stood there, even with bubble, would go down quicker than a commando.
Sages played correctly can be beasts.
Another tip for survivability, experiment with your cleanse. You can cleanse ask tech stuff so do it. That means when vanguard's or gunslingers put a dit on you cleanse it.