Hey, you’re 2*welcome 
I don’t think it is possible to completely remove all the antennas. If you want hardware kill switches, I suppose Librem 5, or just put your device in permanent airplane mode.
It seems to be possible to get rid of at least some antennas
There are some people who have trouble with LTE connectivity. You could probably put something in between the connectors on the other side of the display to at least jam that signal.
Once I got my FP3 I might start experimenting with this. Although I don’t want to damage my FP2, and my plan is to sell it after a couple of weeks.
Not sure if hardening of USB port is required. The other attack I’m aware of, is TEMPEST.