You misspelled “convenience implications” 
(some olden secondary device doesn’t have to be costly)
I mean, if you really care, you have of course two machines.
One for the Internet and one for Data.
And possibly, no USB stick will ever know both devices.
It DOES have cost implications if you use two Librem laptops for that - it’s for a reason you can order them air-gapped (without WIFI) 
Of course, you would never have them both turned on at the same time - or otherwise, everytime your Librem unexpectedly has it’s fans full blast, you’d be very worried that it tries to exfiltrate data from the air-gapped computer by modulating fan speed 