The non- Apple cable I was using with the Anker USB brick WAS killing charging performance as proven by the testing I did which is documented in the "Long Answer" section which follows
The Apple USB-C/Thunderbolt cable would require a USB2.0-to-USB-C adapter to work with my Anker USB brick, so I just bought a brick with native USB-C support which works great:
Zendure X6 45W 5 Ports USB-C Hub Portable Charger 20000mAh PD&QC 3.0 Power Bank
HOWEVER: I remarked in the comments in the Apple store that the Apple USB-C/Thunderbolt cable received many 1-star reviews; not robust. Coiling & uncoiling this cable seemed likly to damage it, so I did some searching on Amazon and found the following cable, which works well with the Zandure X6 Brick from my (short) experience of it:
Anker USB-C to USB-C Thunderbolt 3.0 Fast Charging and Data Transfer Cable
Now my brick runs flat first before the Macbook's battery begins to run-down. Problem solved.
Do remember to shut-down any unnecessary CPU-intensive tasks while running on a USB brick or you will watch it drain fast. And I do mean "FAST"
If you want to see how I arrived at my results, please continue reading "Long Answer" section. Lots of pictures to support my analysis
Since my Macbook charged correctly using the supplied 87 Watt Power Adapter and USB-C/Thunderbolt cable, I needed to assess the charging properties of this connection to obtain a baseline of what "normal" performance was. But how?
Task #1: Identify Appropriate Power Meter:
I'm not an electrical engineer, but searching in Amazon I found the tool for the job:
Jokitech USB Type C PowerMeter tester
Task 2: Identify baseline performance of a correct power connection
Armed with the Jokitech tool, I measured the properties of the stock Apple 87 Watt Power Adapter and USB-C cable that ships with my Macbook Pro 2018:
Stock Apple 87 Watt Power Adapter using Apple USB-C Thundebolt Charging cable
The voltage fluctuated between 19.7-19.9 volts, so we'll use the median number 19.8 as a baseline for assessing performance of other USB cables.
Task #3: Determine charging performance using a USB Brick with non-Apple cable
This test I swapped-out the Apple 87 Watt Power Adapter with an Anker USB brick to power my Macbook
Anker 26800 mAH USB Power Brick and USB 2.0-to-USB-C cable
The voltage of the USB Brick's cable is barely 1/4 of the correctly charging connection using stock Apple Power Adapter & USB-C/Thunderbolt cable.
But is it the USB Brick or the Cable- or both?!?!
Task #4: Compare Apple USB-C cable with non-Apple cable
The Anker Brick I had been using had no USB-C interface, so I found the Zendure X6 45W (5) Port USB-C Hub Portable Charger 20000mAh PD&QC 3.0 Power Bank which enabled me to test both the non-Apple cable reporting low voltage and the Apple USB-C/Thunderbolt cable with the same USB brick.
The first test shown below with the non-Apple cable reporting low-voltage, still demonstrates same poor 5-ish volts performance with the new brick when connected to the USB-C port via an adapter:
Now, if I connect the Apple USB-C Cable which was known to work correctly with the Power Adapter and it continues to operate at about 19.8v (or thereabouts), then that cable I was using with the Anker was causing me the pain. Let's see how tha Apple USB-C cable behaves:
Note that there's no issues with low voltage with the Apple cable. The original cable I had tried with both the Anker & Zandure X6 USB bricks performed 5-ish volts consistently. As you can see, the USB cable you use to charge from a USB brick can have a HUGE effect on charging performance