No, we need a way of building hardware from scratch using simpler components that is very difficult to insert usable back doors into.
Is that technology available already? Is it produced at scale to satisfy a big-enough demand if tests show that it's better than what hardware wallet brands normally use? It's only theory I think.
How can it be hacked if it's not connected to the internet and if the seed & all user data gets wiped when the device shuts down or reboots?
I don't know. Hopefully, it can't and never will be. Remember the saying,
"where there is a will there's a way." BobbysTransactions mentioned one possibility in his post just above yours.