IIRC, your 3 units were early ones? I've got a recent version (SN: '06' and Lot: J045) - seems likely I'm BootRom disabled.The bootrom only comes on for a second. You need to run a script to stop it. If you are seeing phone you have access. Lab126 is different. The bottom was disabled about March 2020. I have successfully shorted this, posted earlier in this thread. Shorting isn't needed though. Volume + with the battery unplugged, then plugging in to USB gives you access, providing your device is older. Tested on 3 units.
When I use @k4y0z 's FireIso I don't even see 'Mediatek Phone' anywhere. And I can't get your battery unpluged / volume+ method to work - even using Suez (only with battery in will a button press combo get me into BootRom on that device). What is the the 'script to stop' you mention to access BootRom? Maybe that's what's missing for me (I've not come across that in the thread - can you help me with a link?)