Hi, I have experienced both MG and thyroid weakness.
Hypo and hyper thyroid weakness, the hyper first because of Graves, later because of too much t3.

A very simple description ( my own, not scientificly or something):

Hypo thyroid weakness: heavy feeling, feel slow and cold. Everthing just feels slow; bowels, thoughts, bloodstream, heartrate, everything.
Hyper thyroid weakness: a weakness that feels like it is caused by overdoing it. Like i've run a marathon without any training. But also other things like anxiety, heartrate even in bed over 120, etc. It is more an overall weakness.
MG weakness: overall weakness, combined with typical weakness: when I think about it, I can say: my weakness is now most severe here and here. For example: now it is more my breathingmuscles and neck. This morning it was mor my mounth and hands, etc.

So with thyroid caused weakness my weakness was never the first or most obvious symptom, wereas with MG it is. With the thyroid it was just like an overall over- or underload. So with co-excisting symptoms.
The same with reumatoid muscleweakness: it is more an overall weakness and not my first, most prominent symptom.

However, since you have more AI diseases, they really need to look into this...

Anyway, my worst weakness is defenitly with MG, so too extreme to be MG is the most retarted thing. I mean, it means severe weakness.... It's in the name! The thyroid weakness (and my thyroidproblems were so severe I had to be admitted, had cardiac arrest, and my bloodtest were too high and too low to be measured even) never caused me to need breathingsupport.