With the release of the Explorer Edition, is there or are there going to plans to update our modem to a 5g modem, or should I just save money to get the newer phone?
Which newer phone? The PinePhone and the PinePhone Pro use the same 4G modem, a Quectel EG25-G. PINE64 has not released any 5G cellphone, and I am not aware of any plans to release one at this time.
What is sure that upgrading the modem if and when a 5G model will be out will require replacing at least the mainboard if not the entire phone, because the modem is soldered on the mainboard. But waiting for a 5G model does IMHO not make any sense at this time, since there are no public plans for one.
Probably nobody wants this, but shouldn't it be possible to make a battery-case-like wraparound thing with a 5G modern in it?
:wq
Most likely yes, or even a backcover replacement like the keyboard etc., or you could just plug a USB modem into the USB-C port (even the built-in modem is a USB device after all), but it will also need software support. And the speed will be limited by USB 2 speeds in all scenarios.