Is a free antivirus actually good enough for a home laptop?

Free is fine, but switch on the two things Defender doesn't always enable. Windows Security > App & browser control > Reputation-based protection, turn on Potentially unwanted app blocking and check SmartScreen is on. Those catch the bundled junk installers that cause most problems on home laptops. A password manager and two factor on your email do more than any paid suite.

On Windows, yes. The built-in Defender scores well in the independent lab tests. Paid suites mostly add extras like a VPN, not better protection. Keeping things updated matters more.