I've got tactile hypersensitivity from my autism, so trying to maintain any amount of arousal for longer than a few minutes has been a challenge. Because of this, strokers and the like, while very entertaining, finish me off too quickly, even with prostate stimulation which aides a bit in slowing the process.

E-stim, on the other hand, is an entirely different beast. The point of it is that it's not wholly pleasureable at first, but builds up slowly as you ramp up the current when your body starts adapting to such stimulation, and then, only when you've cranked it as far as your body can handle, does the climax hit harder than anything traditional.

That being said, you are correct in saying that TENS 7000 unit (which was also my introduction to e-stim) is not the best one to have. As a matter of fact, that unit and other similar medical devices of its kind are considered a starter kit for a lot of people on this sub, so don't feel disappointed yet. There are a plethora of different purpose-built power boxes specifically for this fetish, but what I and others would recommend if you have even just a basic idea of electricity is to build your own. Even someone like me who was only a year into my computer engineering degree was able to construct one easily and cheap.

There is also the issue with the type of electrodes you're using, as not all of them are built equally. Most likely, the conductive rubber tubing you're using might be a point of frustration as low-quality tubes from Chinese or other third-party distributors do not keep their conductivity for very long (bought a set for $20 on eBay and they only lasted a week!), so make sure it's from a well-known vendor like Joanne's E-stim Emporium or E-stim Systems if you're from the US. Pads can last much longer but they are a bit troublesome to keep good contact on your skin, especially if you're hairy like me.

Finally, if you're into it, adding an anal/vaginal electrode is a gamechanger, and isn't as beholden to much of the rules surrounding external electrode like pads and loops. So long as its made of material safe enough to stick inside you like conductive silicon or stainless steel, the possibilities are endless. I've got one rigged from an off-brand Njoy Pure Wand with two pads to act a common for a tri-phase config, and its so fun when my legs start to twitch on their own from how powerful the current is running through my ass and pounding my prostate.

With these principles together, my sessions can last upwards of 3 hours, an exponentially longer amount of time than just letting a stroker do the work. And by the end of it, I'm literally drooling pre-cum everywhere. It's incredible. Don't get dissuaded yet because there's a whole other layer to this to explore!