Sorry to hear you’re still struggling with heart palpitations. Have the changes you’ve tried helped at all? I think there’s quite a few people lurking in these threads that include some small amounts of plants for their own varying health reasons
strict carnivore’s a good starting point but everyone’s individual needs are different and we can adapt as we learn what works for us. I remember (I think it was you) you struggling with what you thought was oxalate dumping, and taking a lot of magnesium was helping you back a couple of years ago. I’ve probably given too much info here, but I wanted to add my journey of coming off strict carnivore to heal my heart rhythm issues. I can concur that some mindful carb inclusion has helped, whether it’s the actual carbs or how those carbs influence serum electrolyte levels I don’t know. I also take magnesium glycinate still and notice my heartrate is slower and more stable when I do.
I started suffering from episodes of tachycardia and heart palpitations this summer. I had been back on strict carnivore for a couple of months and it was triggered by drug, medication and alcohol toxicity (hence why I’m six months sober) but the full on chest pain and afib was terrifying. Reading your post about the pain you’re still having made me clam up a bit.
Like @Jimbob, I also had to increase my carbs to settle my heartrate down. After I went totally sober (caffeine was triggering both tachy and palpitations, as well as worsening my electrolyte problems on carnivore, I ended up in the emergency room convinced I was having a heart attack after having a diet coke
), I stopped having severe episodes but still had some tachy and palpitations every day; my heart rhythm didn’t stabilise until I added mashed potato and peeled apples to my diet.
I kept the rest of the carnivore stuff, meat, butter, and goats milk, but still got bouts of chest pain and palpitations until I came out ketosis and aimed for 100-150g carbs a day. The reason I did mashed potato and peeled apples and like 1-1.5l goats milk as my carb sources is because my gut was inflamed from the alcoholism, and any signficant source of insoluble fibre triggered gut spasms and I was able to link the gut spasms to heart palpitations and spikes in heartrate.
After about 6 weeks on ‘higher’ carb, my system seemed to calm down and I stopped having heart episodes except for the occassional spike in panic related to med withdrawal (a separate issue) and I was able to go back to keto. I now get 18g carbs from 300ml goats milk (high in potassium) and supplement magnesium. I am too scared that my heart issues may return if I go lower carb or cut out the goats milk, as I am too scared to risk low potassium or magnesium issues, so no experiments for me for now, while I can I get my potassium from an animal source at home and remain mostly carnivore (I eat a small amount of cucumber, courgette and leeks as the fibre seems to help my gut and prevent gut spasms which can still trigger palps for me), if I travel I will eat more green veg for potassium or take an electrolyte supplement.
That was a super long winded way of saying I agree that including some carbs may help. If you want to remain carnivore, I highly recommend full fat goats milk as a super source of potassium (200mg/100ml and 4.3g carbs/100ml, it also contains a source of animal fibres, oligosaccharides, that our guts can ferment into fatty acids to boost ketone production). If you want to remain in ketosis you can add up about 500ml, for 1000mg potassium and 22g carbs. When I was in my acute healing phase and not restricting carbs I drank about 1.2litres a day, for 2400mg potassium and 52g carbs.
I hope you find some relief whatever you try 
Edit: Also, on coffee addiction. The only thing I finally found that weaned me off through the cravings was roasted dandelion root, ground up using a coffee blade grinder, a tsp powder brewed in an aeropress (using the inverted method so you can leave it to brew for 2mins) like you would make a coffee, then pressed into a mug with a heaped tsp of melted butter. Viola. 0%caffeine “coffee”. A bit of a faff to source the stuff, but it got me out of my addiction!