ב''ה, would figure there's got to be a relatively painless (aside from any side effects of the heart, y'know, stopping) way to go about this with an actual 'fibrillator' device similar to TENS, and I've said as much on here, while wall current/voltage/waveform sure is as tingly and potentially paralytic as others have described with side effects of that's a lot of tingle in a sensorily unpleasant way and, one would assume, burning flesh if you fully short yourself to a return path (neutral/ground) where I'm not sure the tingle would in any way override the pain of the burn.
So that's really what's up playing with that kind of electrical, while worth noting ECT in its early days literally was just current limited wall electricity so experience reports of that would be what it may be like to attempt to run that directly through the brain and all intervening tissue and bone. No idea how much nuance the modern equipment can really add to that, I've no familiarity but gather they've gotten a bit more thorough with the 'current limiting' part and have played with alternatives to just the standard AC waveform possibly mostly based off intuition and 'so did that work as good and hurt less?' trial and error kind of stuff.
So as noted previously, the thing about stuff like TENS is, it's apparently decent at making muscles contract, while consumer stuff is possibly intended to be a bit hard to unintentionally stop your heart with inadvertently, so then would sinking electrodes into your chest while trying not to endure the pain of hitting a lung be at all pleasant, or would attempting to place the contact ones from a stick on electrode unit to make a path through the heart in what's ordinarily the exact 'do not try that because it would be bad for your health and continued survival' be sufficiently low in pain when switched on for what it might do?
Basically it would be nice if this were a solved or known problem if considering any of that, as much as there surely was plenty of experimentation on lab animals in the past and even contemplating doing that research over just so it could have human use is a bit off-putting if it's already been done in the development of what became the more life preserving defibrillator.
For what it's worth my understanding is that to even be a reliable mode of state sponsored death, the electric chair in practice ended up using 'a bit more than 110 volts,' probably more like 480 or something (specs are available easily enough online) because trying plain wall current even through the top of the head out the feet or wrists with the equivalent of conductive gel wasn't reliable enough with 110-120 or pleasant (potential for lots of burning but not necessarily lethality, rapid or otherwise), so that's a thing to keep in mind.
The other thing to keep in mind is that it's a bit easier for wall current to paralyze muscles and feel as described (a massive amount of 60hz or 50hz tingling of your nerves in the extremity touching it to an uncomfortable extreme, and that's when not particularly grounded to anything to potentially burn yourself with massive amounts of current and just acting as an antenna for the electric grid while insulated by rubber soled shoes and not completing a circuit), so that may make it hard to pull away and can become a risk for anyone grabbing an electrocutee as well. If not doing this as a final act, just playing with touching a wall socket, if it gets you up to the shoulder as it generally may it takes a twist of the hips/torso/portion of the body that isn't 'stuck' fairly paralyzed from the contact to get your finger away, and of course if that nudges you into completing the full circuit path between hot and neutral on the plug you may have a very burnt or missing fingertip after all that.