You can get the sim card in and out of an electron soldered to a device, even if it is soldered all the way to its spacers. You just need the right tool – I use a solid core 26 gauge wire with a hook on the end to do this because I haven’t found a plastic pick small enough…
You might check out what @pNrie has done here for more reference on electron outdoor boards: Anybody using a particle device with SDI-12 interface?. His connections for antenna and battery work well the battery connector is more rugged than the JST header.
On your concept for how this board is used – is the idea to make this board and then plug your other i2c sensors into it? I am guessing this board and those sensors will be ruggedized? What kind of enclosure are you planning on using?