Yes, that makes sense, the 8 is a nice optimum. But if you engineer for maximum reliability and still need more than 3 gears then that five is as good as it gets. It's the NEXUS SG-C7002-5CD 5-speed model, to be specific. There is also an e-bike one for even more torque applications.
IMO for e-bike you probably should be looking at the enviolo CVTs. Sure they're a little less efficient but you can get 330% gear range with ridiculous durability and have the option for electronic shifting. (that said my bike is a non-ebike shimano alfine 8)
I bought a enviolo CVT about 12 years ago because I happened to find one unused a cheap on eBay. It was so heavy once I handled it and the shifter seemed cheap so I didn’t even try to build a wheel with it. I could barely give it away, ended up trading it for a tune-up.
Because it eats up range like there is no tomorrow, makes all kinds of weird sounds, you need to keep it taut on the shifter to get to the highest gear ratio (it will 'unwind' all by itself if you don't do that), and once the batteries run out the bike is so heavy to cycle you may as well give up. And this is as good as it gets, nicely lubricated, regulated as good as possible. It's a very interesting idea but it is either not executed well enough or it is inherently problematic and it won't get better. Buying that bike was a serious mistake, fortunately that wasn't my decision because I'd feel miserable. CVT on a bike is a very nice feature and there are some really interesting developments in that space, but Enviolo isn't it, no matter how much it gets pushed.
they are slow and heavy. Specifically they are roughly 85% efficient compared to ~90% for an internal gearbox or ~95% for a derailer. They also add about 1 pound compared to a gearbox or 2 pounds compared to a 1x derailer system.
That depends: if you are at the edge of your range then the Enviolo may well cause you to have to pedal the hard way with a bike that feels as though you're going uphill all the time. I only see one positive about the Enviolo and that's that you can change to low gear at a traffic light even when you're stopped.
He actually recommends Shimano Alfine 8, he finds it has a good balance between robustness and the way it handles.