It seems that the WITRN C0+ (description here: https://www.witrn.com/?p=952) has an Ah counter, among other counters. At the moment I do charge the power bank itself from a power supply. I have to look/learn how to reset this counter, though.
I have another question: sometimes (not always) when I plug-in the USB-C cable into the L5, on the other end is already connected the USB-meter and the A1281 porwerbank (see photo) which is full, the red led on the L5 is only blinking a few time (up to 30 secs) before going on stable red and with this the USB-meter goes on as well at the same moment, i.e. power is provided. Is this some kind of negotiating of the protocol of both devices?
I think that you are there where you want to be (at the finish line), even if this not for certain (as I’m not some expert). Seriously, what I can see on related picture is that your Librem 5 battery have chosen to be charged with D+ of 0.07V and D− of 0.07V (reaching almost 4.20V, or through another viewpoint reaching its max. capacity, just my distant guess based). My related thought, at this moment, is that D+/D− is more related toward which kind of power supply unit you are using to charge your here related Linux phone and that this resulting handshake (negotiation) comes from your power bank at the first place (best charging option/protocol at 5V selected or even the only one available, telling us that pure PD power supply used there as source).
Just my two cents and as linked earlier, probably ideal charging protocol at 4.50V to 5.20 volt range might be closely related to 0.00V/0.00Vfor D+/D− (or rather equals to those values toward the end of actual charging cycle, of some Li-ion battery), so I’d consider this type of charging your Librem 5 battery as perfect, IMHO. For example, one of my USB-A based power banks just finished charging my Librem 5 with D+/D− at 0.20V/0.20V (under very same charging protocol as on your picture shown on another, yet quite similar USB-C meter you’ve used here for this measurement).
And as I just found in my archive, this would be demonstration (while D+/D− registered at 0.085V/0.085V) of what I’ve tried to explain within above sentences, under both sides (source<=>sink) negotiated or rather both sides triggered (choice of this word should be considered as correct here) PD2.0 charging protocol (and I hope that this approach, as shown, helps you further building some conclusion of your own):