So glad you are feeling better...being able to use a modern Mac is fantastic.
degen Add in muscle impairments and modern flickery technology and you have a recipe for disaster.
Martin has a theory about this. Not sure if you optometrist tested for suppression, but it is common in folks with BVD. The brain ignores the input from one eye, but you are not conscious of this.
The flickering forces the brain to use two eyes -- in fact, to treat suppression, a vision therapy exercise is to use flickering light, similar to PWM. "Treating" suppression before fixing binocular vision problems can cause double vision and is a danger of "self-prescribed" vision therapy.