Is it pretty common to have normal tsh level (levothyroxine 12 yrs) but still be symptomatic?

Yes: Thyroid levels does not take into account the peripheral conversion of t4-t3 sometimes T3 (liothyronine) at tissue level would be low. I found environmental pollution and increased plastic usage thyroid requirements are higher. People who has muscle issues - myopathy has higher thyroid need.

Thyroid : Yes. The "TSH" is actually produced by the pituitary gland to stimulate the Thyroid to produce T 3 , T 4 , which negative feed back to the pituitary gland . The T3, T 4,T 7 , as well as the Thyroid Antibody needs to be analyzed , because a normal TSH , is not diagnostic unless it is analyzed with T3, T4, T7 and antibodies at the same time.