Short answer
Much safer than plaintext on cloud storage, but far less safe than offline physical backup. Security depends entirely on password strength: 7z and VeraCrypt use AES-256, which is mathematically sound, but once an attacker has the encrypted file, they can brute-force your password offline indefinitely — no rate limiting, no login throttle. If your password is "birthday + name," it falls in days. Acceptable strength is a 20+ character random string, and that password cannot live on the same cloud.
The "offline brute-force" reality
Many people's sense of security with encrypted archives comes from the wrong intuition: "without the password, nothing opens." This is true if your password is strong. But most people's passwords are not 256-bit random — they're 12-16 character mixes of common words and birthday digits.
The key concept is offline brute-force. While the encrypted file sits on Google Drive, an attacker needs to compromise the Google account first — and Google has 2FA, rate-limiting, and unfamiliar-location alerts. But once an attacker phishes the Google credentials or hijacks a cookie (as happened in the LastPass 2023 breach), they immediately download all suspicious files locally.
After local download, cracking is pure compute. No throttling, no alerts. A single RTX 4090 running hashcat tries millions to tens of millions of password candidates per second. A weak password falls in hours.
What "sufficiently strong" actually means
For 7z AES-256 to be resistant to a state-actor-level attacker: 20+ character random string, drawn from a 70-character alphabet. That gives roughly 122 bits of entropy. Memorizing this is impractical for most people — and writing it down means you now need to protect two things. The encrypted-archive approach effectively reduces to the same problem you started with.
The recommendation
If you must use cloud storage as one tier of backup: use VeraCrypt with a passphrase generated as a Diceware sequence of 7+ words. Do not store the passphrase on the same cloud. Store it on a separate device, ideally on paper or steel in a different physical location. For most US holders, this is more operational complexity than steel-plus-safe.
Further reading: Five storage methods.