I don’t know that I have direct answers to your questions, but can certainly empathize which what you are experiencing. I suffer from an arrhythmia which takes the form of palpitations and irregular heartbeat. It’s not symptomatic of an underlying heart condition (I’ve been tested several times by a cardiologist),but it is highly uncomfortable and makes me anxious, which my brain can’t always talk me out of. I did first have palpitations when I was diagnosed with hyperthyroidism. I had my thyroid destroyed with radiation and now take thyroid replacement medication. My thyroid has been stable for years and that doesn’t seem to contribute to my arrhythmia. After being stable on a dose of synthoid for many years my thyroid went out of whack after I was diagnosed with type 1 diabetes and we had to alter my dose. So that is something you might want to keep an eye on.
Since I experience palpitations that I know are unrelated to my diabetes it’s hard to separate it out, but I generally experience palpitations when my blood sugar is low, not when it’s high, and that is what I’ve heard from other people as well. That doesn’t mean that high numbers aren’t what causes it for you. One thing we know about diabetes is that it really varies greatly person to person.
I don’t know that I would call post prandial blood sugars of 150-175 “the lower end of prediabetic”. Those are high blood sugars to any of us, though not extremely so. Are you doing something to control those post prandials? Are you positive you are type 2 as having another autoimmune condition such as thyroid is a common sign of type 1 or LADA.
Finally since you are looking at possible relationships between thyroid, diabetes as well as palpitations, you might want to work with an endocrinologist who is experienced with thyroid and diabetes rather than just a primary. Also, it can’t hurt to see a cardiologist just to get any heart condition ruled out.
Sorry if I gave you more questions than answers!