The bark is not a living part, so you may touch it softly.
Rub and plastic are also not ideal (humidity, insects hiding, which could also weaken or damage the inner part).
So it is not easy. You should estimate what it is better for you and monitor your choice, and eventually go back.
On professional orchids I saw three solutions:
a sort of trimmer, but it turns slower, and there are mechanical protections (and springs). This is the preferred way if there are suckers.
a sort of lawn mover (also with mechanical protection and spring). But just cutting below, it offer protection. Having the plants in rows help there.
herbicide. (and I would not comment on this).
Protections are put in place only on the younger trees, where the weights of springs will be too strong.
Personally I use either the lawn mover: much more quicker (often a small old one, just to go under the trees), and a string trimmer. I would use the trimmer on lower speed near trunk, and if I hit strongly, I would really try to be much more softer on the other sides of the trunk. My string trimmer has also the kit for blades, and so a larger protector (that just fit the string). This help me a lot on getting the distances (and I can use it, on the side, as protection, not to get to near to the trunk). Note: shadow and roots help me (on my climate) to get less strong grass near the trunk, so the "precision trimming" is done twice a year.