Nobody should be using an online password manager;
especially when it comes to sensitive information like private keys. There is no reason to trusting intermediaries when you have cryptography. Install
KeePassXC on both your main computer and your mobile. Use a strong password to encrypt both password databases. Back them up. Both the databas(es) (digitally) and the encryption pass (on paper).
I never used LastPass, I was actually very surprised that their app doesn't let their users store their data locally. Or do they, and online storage is merely one of its features for convenience and accessability?
But OK, I would probably use something like that for hot-wallets containing small amount amounts of Bitcoin for playing Craps in a casino, but never for storing my Bitcoin life-savings.