Keep in mind that a Factory Reset and flashing firmware are software-only solutions, they cannot solve hardware-based problems. Also stop wasting time trying different drivers and other such utilities, that's all software. And given you've tried different computers that are running completely different operating systems that's a dead end too. At this point, there's most likely to be some kind of USB-hardware related problem with your phone. Again, software can't fix that.
https://www.gsmarena.com/lg_l70_dual_d325-6101.php
It's an eight year old phone, really dated as far as smartphones in general so it is not too surprising if some internal component does fail. Although it could be something as simple as cleaning the metal contacts inside the USB port. There are two dedicated solely for data transfer and two dedicated solely for power transfer. In your instance perhaps one or both of the metal contacts for data have just built up some grimy corrosion over the years.
https://pinoutguide.com/PortableDevices/micro_usb_pinout.shtml
But a phone this old isn't going to remain functional forever so work on this as challenging project but don't get too fixated.