Is it ok to store pv-keys and seeds on a usb-stick,
and before you use the stick just disconnect from the internet?
Absolutely not. Your private key (in plain text) and your mnemonic phrase should never touch an electronic device. There is plenty of malware out there which can capture your screen, your keyboard entries, your clipboard, entire files, etc. and hold on to them, and then transmit them to a third party once internet access is re-established. Even having them stored on a device which is permanently air-gapped (i.e. with no internet connection
ever) isn't 100% safe if you use USB sticks or other methods to transfer data on to and off of the device.
You should write down your mnemonic phrase by hand on paper, and then store that paper securely.