A few potentially relevant observations:

• The kind of dissociation you talk about here, where I experience my "self" as unrelated to my body, is commonly reported as spontaneously occurring during various kinds of emotional stress. I've had it happen to me many times.

• It would not be surprising if the same mechanism that leads to spontaneous dissociation in some cases can also lead to the strong intuition that dissociation would be a really good idea.

• Just because there's a mechanism that leads me to strongly intuit that something would be a really good idea doesn't necessarily mean that it actually would be.

• All of that said: after my stroke, I experienced a lot of limb-dissociation... my arm didn't really feel like part of me, etc. This did have the advantage you described, where I could tell my arm to keep doing some PT exercise and it would, and yes, my arm hurt, and I sort of felt bad for it, but it's not like it was me hurting, and I knew I'd be better off for doing the exercise. It is indeed a useful trick.

• I suspect there are healthier ways to get the same effect.