I am more familiar with approaches to dealing with depression than anxiety/panic but I do have some thoughts.
You have a very tricky situation there with the a-fib, which can be indistinguishable from a panic attack by its symptoms, and which can also be triggered by anxiety/panic. I can tell you I had panic attacks for awhile that were caused by physical heart-related sensations. I was having pain in my chest, palpitations, rapid heart rate, and trouble breathing, and these symptoms would provoke a panic attack. This went on for some years and I was treating it off and on with psych drugs. Eventually, I figured out the physical symptoms were just reflux: a combination of GERD and LPR. Once I understood that as the cause, and knew at a deep level that these symptoms were not life-threatening, they lost their capacity to provoke panic attacks.
In my mind, to sever the anxiety/panic response to physical symptoms, what you would need is to be fully and deeply convinced that these episodes of rapid heart beat are not posing a serious threat to you. That is easier said than done, especially if there actually is an element of danger, if these episodes are sometimes a-fib or you are in danger of triggering a-fib by having them. I guess I would be exhausting all possible diagnostics, hopefully performing some during the episodes, to prove to myself that the episodes are not dangerous. I would also be reminding myself that I had not died yet despite this occurring so many times previously, so this next episode I'm experiencing is not going to kill me either and will pass just like the others.
In terms of what else you could try, I'm sure you've done obvious things like magnesium. Some research has shown increased risk of a-fib with omega-3 supplementation:
Omega-3 Fatty Acids and Atrial Fibrillation
As I recall your diet is very heavy on the fatty fish. I've personally experienced anxiety symptoms from higher doses of EPA. I would try replacing the fish with some different meat for awhile if you haven't tried that yet. I would also experiment with carbs along the spectrum between moderate carb and full keto. That may have some impact on your symptoms for good or ill.
I would also be suspicious of the thyroid supplementation. I mistakenly thought I was hypothyroid some years ago when I was following a poorly executed low carb diet without enough fat, that left me cold and fatigued all the time. I began supplementing with armour thyroid and saw a clear relationship between taking too much thyroid hormone and anxiety symptoms. I felt best when I was not taking any. That may not be an option for you, but a dose reduction might be.