This is the pathophysiology I had been taught and the clinical signs I have been instructed to look for. Double checked with National Institute of Health and it also talks about ventricular dysrhythmia/arrhythmia (specifically torsades de pointes, so I guess maybe I should have been more specific and said polymorphic ventricular tachycardia), and various autonomic and cardiotoxic effects. I do admit I was sort of of the ball, and it seems there is more focus on blockade of autonomic receptors. As far as the cardiac action potential goes the main effect is on sodium channels and not calcium, so I was mistaken there too.
I can guarantee to you that where I live our first-line treatments for TCA OD are norepinephrine, sodium bicarbonate, magnesium sulfate, and calcium gluconate. I'm not going to send the protocols to you because I would essentially be doxxing myself, but that's what we do. Even if I edited out any organization names, the flow charts and dosages are available online (our protocols are public access), so I'm sure it could be traced back to my jurisdiction and people could find out where I work. Norepinephrine is administered as part of our shock protocol, so you could also include fluid resuscitation and epinephrine or other pressors.
When I talk about ODs in this context I'm mainly talking about the type you mentioned where people take acetaminophen, diphenhydramine, benzodiazepines, etc. Of course a well-planned overdose can be fatal. People discuss pentobarbital and fentanyl on this site. Pentobarbital is pretty much impossible to get, but some people claim to have done it by going to places like Peru or Bolivia. Fentanyl is the same, street fentanyl is unreliable. You can never be sure of the dose or concentration, and pharmaceutical fentanyl is only really given for home use in patch form. I'm not sure how you could make that work lol. There just wouldn't be enough, even if you had no tolerance and used multiple patches. I think patches generally release something like 100mcg per hour, but we literally give that as a slow IV push for pain or sedation prior to airway management. So over an hour even 5 or 6 patches probably would just sedate you but do nothing fatal, even if you had absolutely 0 tolerance. I think the best way would be a MAID death: midazolam, propofol, rocuronium. Propofol is impossible to get though. Not prescribed for home use, never heard of it being sold illegally. Rocuronium would probably be the same. No recreational use so it wouldn't be on that market, and also not prescribed for home use. I'm not familiar with lethal injection but I've heard that they use potassium for that. I suppose this is an option as well, but I doubt it would be pleasant and again, it's unobtainable. No recreational purpose, not prescribed for home use. So you're left with other prescription drugs. Possible? Yes. Would it be as peaceful and reliable as a midazolam-propofol-rocuronium or pentobarbital overdose? I doubt it. Again, in my experience when people take beta blockers, calcium channel blockers, TCAs, or other drugs, generally they do not die unless not found for hours. I think peak toxicity is around two hours or something like that. But honestly most cases I have seen have been people calling 9-1-1 for themselves because they change their mind, the pain/symptoms/discomfort become too much, or they have/get survival instinct. I suppose there's probably observation bias here though. Last thing I can think of is an insulin overdose. I've never seen it myself, but I've heard of people taking all of their insulin at once as a way of causing hypoglycemia. This is obviously a cause of arrest, so I guess it's possible too. Don't really have much experience with it though so I'm not going to speak to it's efficacy as a way of CTBing. I suppose there's also things like xylazine or nitazenes, but I have pretty much no knowledge of them so I'm not even going to say anything. I'm pretty sure they're not sold illegally as their own thing, but I know sometimes fentanyl or other things get laced with it. To my knowledge xylazine is only used in veterinary medicine and I doubt it's given as a take home, and I think (?) nitazenes may be used in humans, but again, I sort of doubt you would get them without being directly given them in a clinical setting by a healthcare practitioner. It's possible, but every drug accessible via take-home prescription or OTC is not something I would really consider a preferred method of CTB. I think a lot of people on this site want a peaceful death, hanging is reliable but a lot of people don't want to do that. You could overdose, but a peaceful and reliable one that I would like to die by is pretty much out of reach.