This is a lot helpful. But you said, it is inconvenient. It is also can not be safe like the recommended HW wallets.
If your only intention is to keep your private keys offline, I would argue that it could be just as safe, since it does exactly the same thing.
One of the main advantages of hardware wallets, in addition to keeping your keys permanently offline, is that if they fall in to an attacker's hands your coins are still safe (or at least, safe for long enough for you to recover your backs ups and send them to a new wallet). Your set up misses out this important protection, unless you are also encrypting the USB drive.
Strong wallet passwords could also help, so it's not
entirely vulnerable in the hands of a potential attacker. I mean, we've all heard about people getting locked out of their Electrum wallets, so you can possibly make this work in your favor. Hardware wallets are definitely better in this area though.
One advantage it has though, is the attacker won't necessarily know the flash drive is holding coins (you could just be using it as an OS installer after all, like majority of the populace) unlike hardware wallets, so they could be less prone to thievery.