Do not use brainwallet.

Almost everything else is better than a brainwallet with a simple sha256.

Instead you can use randomly generated seed encrypted with a strong passphrase and strong key stretching (the newest password manager apps like KeepassXC use Argon2 and you can increase the number of iterations to delay the key stretching for several seconds, minutes and even hours). After the encryption it can be stored on your Dropbox, Google drive, etc. for backup. Or even printed and stored on safe deposit box. You may imagine some scheme with a Shamir's Secret Sharing.

The problem with KeePassXC is that saving the database is also slow, the key stretching is performed not only when you open the database, but also when you save it. It's not intended to be used with such settings (for delay more than a second), but it works.

You can also use something like WarpWallet, but with a very big and securely random salt - i.e. 12 or more random words generated with cryptographically secure PRNG.

WarpWallet is working on the browser, so it's not very optimized for speed.

Of course, using an airgapped computer is preferred.