A HAT seems to by way to complex and less flexible then a small black box you can plug your USB charger in and a USB cabel comes out

Then you have a string of devices that need to be connected together in a more n rigid fashion, either in separate compartments of a bag or case with a USB cable between, or strapped together with tape.

Really? I'd far rather have a UPS hat than another external brick, assuming the hat provided passthru so it didn't block the addition of further hats.

And indeed that one does:

"PiJuice will use up to just five of your GPIO pins (just power and I2C), the rest are free to diversify your project. The stacking header allows you to continue to use your existing HATs and add-ons with PiJuice." from https://uk.pi-supply.com/products/pijuice-standard

But sure, some people/projects might prefer to shift the hardware to not be attached to the Pi. I wouldn't be surprised if you could find a product that does that too, or at worst I presume a basic plastic box (there's probably pre-made ones for the size of various Pi HATs? and of course 3D printing) and some fairly simple DYI wiring to connect the 5 pins through a cable wouldn't be too tricky.

I used one of these for about a year on my PiHole, but switched to a standard UPS because the hat's LiPo started bulging from all the heat generated by the Pi's processor.

The hat is a great idea in principle, but because the battery is sitting right on top of the processor, it doesn't stand a chance given that lithium batteries are fairly temperature sensitive.

I'd go with another option.

Good to know, thanks!