I bought a couple of these:
https://www.aliexpress.com/item/32896041237.html
Best thing since sliced bread. I have a USB device that measures power output/input, and they will output the advertised power (5V @ 3.1A, 9V @ 2A, 12V @ 1.5A, etc.) if the device supports it. USB-C port functions as an input (for recharge) or output, so you can use a USB-C to USB-C cable to charge things, in addition to standard USB-A to whatever cables. The display will actually tell you whether it's outputting at 5V, 9V, etc.
The best thing about it is you have to supply your own 18650 cells, so you know exactly what you're getting.
Before these, every "power pack" or whatever fancy marketing name they're given I've ever owned has always contained cheap Chinese cells of dubious quality and about 1/4 of the rated capacity. Most never lived up to their advertised output; a couple claimed 2.1A output, never got over 500ma. I was lucky to get a year or two out of them.
But these suckers are something else when paired with 7 18650 cells. My wife's has Panasonic / Sanyo 2900mah cells, mine has LG 3400mah cells. You can find genuine 18650 cells online; if a Google search doesn't turn up anything helpful, visit some vaping forums, and some old posts will point you in the right direction. The cells were about $5 a piece, so had perhaps $50 in each charger at most.
The drawback is they're heavy (7 cells will do that), but not unwieldy and fits in most pockets. About the size of a large cell phone, but obviously the thickness of an 18650 cell, which most are. Once you snap it together with the cells installed, nothing short of a hammer blow or a nice high drop onto hard concrete is getting that thing apart. Ask my wife who is super careless with such things. In three years she's managed to break one of the USB-A ports (she or one of the kids probably tripped over a cord), plenty of drops from the couch to the hardwood floor, I think the dog tried to get a bite out of it once, three weeks lost bouncing around the car interior, etc. So far we've yet to kill one. Both are working great to this day and charge my phone (Iphone XS Max) 3-4 times before needing a recharge. Will charge an Ipad from 0-100% and then some.