kinaneb

Wow. Those are both major firmware versions that I've never seen. I've added them to my What Axpert Firmware is That? page.
So sorry, I can't help with updated firmwares in this case.

That one looks like it might be a genuine Axpert King II 6.2 kW, possibly a Twin model (two AC output ports).
Indeed, when I scan the QR code, it says "Originally manufactured by Voltronic Power". Of course, the clones could copy that too 🤔

That one seems much like the first one, i.e. an Axpert MKS II clone, despite the name suggesting that it's a VM II (VM II has a smaller DSP and can't be paralleled). It might be by a different clone manufacturer though.

It looks like it's a PowMr clone (possibly with a different label) of an Axpert MKS II 5 kVA with hacked firmware that overloads the output by 10% and the solar charger by 12.5%. Or more, I saw 90 A solar charge current in a PDF, but the label implies 110 A. The hardware is I think designed for 80 A.
IF the firmware can be updated at all, it MIGHT work with Voltronic MKS II 5 kVA firmware, such as 71.86 (beware: not the Axpert King firmware with the same version number), but if it's working at all at present, I would not attempt it.

Well, any inverter has a "no-load" consumption. Typically around 50w, some even as high as 100w.
Also, as you are using AGMs (as opposed to Lithium), they have a self discharge.
You are correct that in SUB mode, loads will only be powered by battery if Solar or Utility is unavailable. But, the self consumption of these inverters seems to always be taken from the batteries.
I see the same behavior on MUST 3K 24V AIO inverter, even in UTI mode, which is basically like a UPS.
What you need to do is to enable Utility charging in addition to solar charging.
You can set it as low as 1 or 2 amps of Utility charging, so that self consumption will be taken from the grid and not the batteries.
 

i also started off with watchpower and found it to be mediocre at best, you can use this multisib software, it is pretty good considering it is free.
 
http://www.multisibcontrol.net/