the set up appears to be an ‘either or’ arrangement, rather than running a light and charging the power bank simultaneously
Yep, I learned this in 2014/15 when I was trying to run an AXA headlight and an Anker powerbank off my SON 28-mit-Sinewave charger. Happily, I was touring in northern Europe st the time, so was able to recharge my power bank every other day, hence the various parasitic devices feeding off that.
I resolved the problem by ditching the AXA headlight in 2015--helped by the fact that its mounting bracket fractured, but the switch was wonky in any case. Since then, my SON 28/Sinewave setup charges only my Anker battery, and does so very quickly: from aboyut 15-20% to full in 3 - 4 hours.
My headlight is an old-but-still-sound Cygolite, which I use almost exclusively on the flashing mode so as to be seen in daytime. Used this way, it can go 3 days or more without a charge. I use it only very occasionally at night.